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Sewage  Purification  and 
Disposal 


By   J.   J.   COSGROVE 


Author  of 

"Principles   and    Practice   of   Plumbing,"   "History  of   Sanitation," 

"Plumbing   Plans   and   Specifications,"    "Wrought 

Pipe   Drainage   Systems" 


Published  by 

$tattdard  jSmtftai®  H)ia.  Co* 

PITTSBURGH,  U.  S.  A. 


Copyright  11)09  by  Standard  Sanitary  Mfg.  Co.,  Pittsburgh,  U.  S.  A. 


Publisher's  Note 

THE  primary  object  of  our  organization  is,  as  is  universally 
known,  to  manufacture  and  market  "<$tQlt<iEttKf  Plumbing 
Fixtures,  Brass  Goods  and  other  products  made  in  our 
factories.  In  the  development  of  an  organization  to  accomplish 
this  result,  there  has  been  established  an  Advertising  and  Pub- 
lishing Department  of  no  small  proportions,  and  "Sewage 
Purification  and  Disposal"  is  simply  the  outgrowth  of  the  work 
of  this  department.  This  brief  statement  will,  we  believe,  serve 
to  give  the  public  a  clear  understanding  of  our  somewhat  unique 
position  of  being  at  the  same  time  manufacturers  and  publishers. 

The  first  serious  work  of  the  Publishing  Department  on  a 
large  scale  was  "Modern  Sanitation"  (established  June,  1904). 
From  this  came  the  publication,  first  in  serial  form  and  later  as 
a  book,  of  J.  J.  Cosgrove's  first  work,  "Principles  and  Practice 
of  Plumbing"  (book  published  December,  1906).  The  phenom- 
enal success  of  the  book  is  a  matter  of  general  knowledge, 
although  it  may  not  be  widely  known  that  "Principles  and 
Practice  of  Plumbing  "  has  been  adopted  as  a  text  book  in  more 
than  thirty  universities  and  colleges  in  the  United  States,  and 
bids  fair  to  be  adopted  in  others.  This  magnificent  achievement 
has  been  accomplished  solely  on  the  merit  of  the  work  and  with- 
out solicitation  on  the  part  of  either  author  or  publisher. 

There  is  now  offered  almost  simultaneously  two  new  books 
by  Mr.  Cosgrove,  one  being  the  volume  in  hand  and  the  other 
"  History  of  Sanitation." 

In  "Sewage  Purification  and  Disposal,"  "Principles  and 
Practice  of  Plumbing"  and  "  History  of  Sanitation  "  we  feel  that 
the  literature  of  the  craft  has  been  enriched  in  an  enduring 
manner,  and  that  we  have  fully  justified  our  appearance  in  the 
field  of  publishers  as  amply  as  we  have  our  standing  as  manu- 
facturers of  a  world-wide  known  and  used  product. 


Standard  cSaititang  IDfij,  Co* 

Pittsburgh,  U.  S.  A. 

Publishing  Department 


Preface 

THE  purification  of  sewage  has  been  a  growing  prob- 
lem in  thickly  populated  centers  ever  since  the 
danger  to  public  health,  arising  from  polluted 
waters,  was  realized.  During  that  period  numerous 
experiments  on  a  large  scale  were  conducted,  and  consid- 
erable data  were  worked  out  in  practice,  so  that  at  the 
present  time  the  principles  of  sewage  purification  are  so 
well  established  that  an  intelligent  engineer,  grounded  in 
the  principles  of  the  science,  can  construct  works  to  effect 
any  desired  degree  of  purification  of  the  crude  sewage. 

Up  to  the  present  time,  however,  no  text  book  or 
treatise  has  been  published  on  the  subject  giving  in  con- 
cise, ready  form,  rules,  tables  and  data  for  designing  and 
proportioning  purification  works.  Further,  the  principles 
and  data  worked  out  by  experiment  and  experience  are 
scattered  through  widely  separated  reports,  public  docu- 
ments and  private  papers,  so  that  they  are  not  in  available 
form.  For  these  reasons,  and  owing  to  the  present  active 
interest  in  the  subject,  it  is  hoped  that  SEWAGE  PURIFICA- 
TION AND  DISPOSAL  will  fill  a  want  in  the  field  of  engineer- 
ing literature,  and  be  a  guide  to  communities  grappling 
with  the  problem. 

The  aim  of  the  author  in  preparing  the  manuscript 
was  to  present,  as  simply  as  possible,  a  work  which  would 
show  the  method  of  constructing  various  types  of  sewage 
purification  plants,  their  details  and  proportions,  together 
with  a  description  of  materials  best  suited  to  the  purpose, 
so  that  any  one  trained  in  engineering  design,  by  following 
the  text,  can  successfully  plan  and  proportion  a  sewage 
disposal  works.  To  this  end  the  illustrations  were  pur- 
posely made  simple,  so  as  to  show  the  principles  of 
construction  without  being  clouded  by  the  numerous 

iii 

187719 


details  of  a  large  complicated  plant.  Perspective  draw- 
ings were  used  in  preference  to  mechanical  drawings, 
so  as  to  convey  to  the  mind  a  true  mental  picture  of 
the  plant,  or  detail  under  discussion.  It  was  assumed  by 
the  author  that  the  designer  would  possess  the  necessary 
skill  as  a  draftsman  to  make  plans,  and  would  simply  want 
to  know  the  size,  shape  and  materials  of  the  several  parts. 

The  drawings  for  this  volume  are  all  original.  No  cata- 
logue or  other  borrowed  plates  were  used,  and  every  effort 
was  made  to  insert  an  illustration  where  it  was  thought 
that  a  drawing  would  help  make  clear  the  text. 

By  an  avoidance  of  technical  terms  and  the  presenta- 
tion of  the  text  in  a  simple  manner,  it  was  hoped  to  make 
SEWAGE  PURIFICATION  AND  DISPOSAL  valuable,  not  only  in 
the  class  room  and  for  engineers  and  architects,  but  like- 
wise to  engineering  and  architectural  students,  municipal 
officials,  sewage  disposal  committees,  and  everybody  else 
interested  in  the  subject. 

J.   J.  COSGROVE  . 
PHILADELPHIA,   PA., 

March  15,   1909 


iv 


Table  of  Contents 

Composition  of  Sewage    .........  1 

Decomposition  of  Sewage         . 14 

The  Septic  Tank 32 

Intermittent  Filtration  of  Sewage 61 

Sprinkling  Filters 97 

Contact  Beds 119 

Sewage  Irrigation     ..........  128 

Subsurface  Irrigation 146 

Chemical  Precipitation      .........  160 

Sewage  Pumping  Plants 168 

Disposal  of  Sewage 176 

Disposal  of  Sludge  .         .         .         .         .         .         .         .         .179 

Disposal  of  Effluents 184 

Disposal  of  Storm  Water 184 

Designing  Sewage  Purification  Plants  ......  186 

Appendix            .         .        . 197 


List  of  Tables 


Page 

I  Contents  of  Urine 27 

II  Free  and  Albuminoid  Ammonia  in  Sewage  Effluents        29 

III  Comparison  of  Results  from  Open  and  Closed  Septic 

Tanks 35 

IV  Effect  of  Different  Rates  of  Flow  Through  Open  Septic 

Tanks 42 

V  Capacity  of  Miller  Automatic  Siphons    ....  56 

VI  Bacteria  Found  at  Different  Depths  in  Filter  Beds     .  62 

VII  Capacity  of  Drains 71 

VIII  Size  and  Rating  of  Sieves 79 

IX  Quantity  of  Sewage  Purified  by  Different  Sizes  of  Sand  92 

X  Mechanical  Composition  of  Sands  Available  for  Filtra- 

tion          93 

XI  Classification  of  Sands 95 

XII  Contact  Filter  Rates .126 

XIII  Sizes  and  Capacities  of  Drain  Pipes        ....      140 

XIV  Statistics  of  Sewage  Farms  in  England          .        .        .147 

XV  Reagents  for  Precipitating  Sewage  ....      162 

XVI  Capacities  of  Centrifugal  Pumps 170 

XVII  Safe  Dilution  of  Sewage  in  Streams       .        .         .        .178 

XVIII  Consumption  of  Water  in  United  States  Cities     .        .       189 

XIX  Population  of  Lancaster,  Pa.,  and  Maiden,  Mass.         .       191 

XX  Population  of   Small   United  States  Cities  at  10-year 

Intervals 193 

XXI  Analysis  of  a  Gravel  by  Hand  Picking          .         .         .202 

XXII  Rate  at  which  Water  Passes  Through  Sands         .        .      212 

XXIII  Rate  at  which  Water  Passes  Through  Gravels     .        .      213 

XXIV  Mechanical  Analyses  of  Sands         .....      214 


List  of  Illustrations 

Page 

1  Septic  Tank 36 

2  Sewage  Screen 45 

3  Mechanically  Operated  Sewage  Screen           ....  46 

4  Detritus  Tank  and  Screen  Chamber 47 

5  Wooden  Shear  Gate 48 

6  Shear  Gate 48 

7  Round  Sluice  Gate 49 

8  Square  Sluice  Gate 49 

9  Sluice  Gate  with  Thimble  Set  in  Concrete   ....  50 

10  Sluice  Gate  Anchored  to  Masonry 50 

11  Sluice  Gate  Bolted  to  Pipe 51 

12  Capstan  for  Operating  Sluice  Gates 52 

13  Capstan  with  Crank  and  Gear         ......  52 

14  Gate  Valve 53 

15  Miller  Automatic  Siphon 54 

16  Miller  Automatic  Siphon 55 

17  Rhoads-Miller  Automatic  Siphon    ......  57 

18  Plural  Alternating  Siphons 58 

19  Barbour  Rotation  Dosing  Apparatus      .....  60 

20  Intermittent  Filter  Bed .         .  65 

21  Cross-section  Through  Filter  Bed 66 

22  Section  Through  Gate  Valves,  Filter  Bed     ....  67 

23  Section  Through  Manhole,  Filter  Bed 68 

24  System  of  Underdrains  for  Intermittent  Filter     ...  72 

25  Tile  Pipe  for  Underdrains 73 

26  Brick  Underdrains 73 

27  Perforated  Tile  Underdrains 74 

28  Split  Tile  Underdrains 74 

29  Nested  Sieves 79 

30  Scales  for  Weighing  Sand        .        .        .        ....  80 

31  Sewage  Distributor 81 

32  Detail  of  Concrete  Sewage  Distributor           ....  82 

33  System  of  Sewage  Distributors 83 

34  Detail  of  Wooden  Sewage  Distributor 84 

35  Aerator 88 

36  Diagram  Showing  Comparisons  of  Sands      ....  93 

37  Diagram  Showing  Air,  Water  and  Voids  in  Sand       .         .  94 

38  Surface  Arrangement  of  Intermittent  Filters        ...  96 

39  Sprinkling  Filters  with  Stationary  Nozzles  ....  100 

40  Section  Through  Sprinkling  Filter          .        .         .        .        .101 

41  Sprinkling  Filters  with  Revolving  Arms       ....  102 

42  Plan  of  Stationary  Sprinklers 108 

43  Surface  Sprayed  by  "Staggered"  Sprinkler  Nozzles           .  109 

44  Surface  Sprayed  by  Tiers  of  Sprinkler  Nozzles   .        .        .  110 


Page 

45  Columbus  Sprinkler  Nozzle      .         .         .         .        .        .        .112 

46  Improved  Columbus  Sprinkler  Nozzle 112 

47  Perspective  View  of  Revolving  Sprinkler      .        .        .         .  113 

48  Detail  of  Revolving  Sprinkler 114 

49  Automatic  Traveling  Distributor 115 

50  Automatic  Revolving  Distributor    ......  116 

51  Contact  Bed 121 

52  System  of  Underdrains  for  Sewage  Irrigation,  Porous  Soil  136 

53  System  of  Underdrains  for  Sewage  Irrigation,  Clay  Soil   .  137 

54  Ridge  and  Furrow  Irrigation,  Level  Land  ....  142 

55  Sluice  Box  and  Flume 143 

56  Ridge  and  Furrow  Irrigation,  Sloping  Ground     .         .        .  144 

57  Catchwork  System  of  Irrigation 145 

58  Subsurface  Sewage  Disposal  Plant 149 

59  Distributing  System  for  Subsurface  Irrigation     .         .         .  151 

60  Method  of  Laying  Subsurface  Irrigation  Tiles     .         .        .  152 

61  Fitting  for  Subsurface  Irrigation 153 

62  Application  for  Subsurface  Irrigation  to  Hillside          .        .  154 

63  Chemical  Precipitation  Tank 164 

64  Electric  Pumping  Plant 171 

65  Gasoline  Pumping  Plant    ........  174 

66  Sludge  Press 183 

67  Diagram  Explaining  Law  for  Increase  in  Population  .         .  192 

68  Diagram  of  Size  of  Sand 203 

69  Diagram  of  Size  of  Sand 205 

70  Diagram  of  Size  of  Sand 207 

71  Apparatus  for  Measuring  Frictional  Resistance  of  Sands    .  210 


Vlll 


Sewage   Purification  and   Disposal 


PURIFICATION   OF  SEWAGE 


PRINCIPLES    OF    SEWAGE    PURIFICATION 


COMPOSITION  OF  SEWAGE 

Conservation  of  Matter — In  the  economy  of  nature, 
nothing  is  destroyed.  The  molecules  of  matter  are  con- 
stantly undergoing  changes  that  build  up  and  destroy,  and 
tear  down  to  reassemble  in  other  combinations ;  but  in  the 
changes  that  take  place  nothing  is  lost;  the  molecules  or 
elements  are  simply  grouped  in  other  forms.  In  the  cycle 
of  changes  that  take  place,  inorganic  matter  is  converted 
into  plant  life,  plant  life  into  animal  tissue,  and  animal 
tissue  back  again  to  inorganic  mineral  matter,  ready  to 
begin  anew  the  endless  cycle.  Vegetation,  in  growing, 
absorbs  from  the  soil  nitrogen  in  the  form  of  nitrates. 
From  the  air  the  chlorophyl  of  the  leaves  and  other  green 
parts  decomposes  carbon  dioxide,  giving  off  the  free  oxygen 
and  assimilating  the  carbon,  which  as  cellulose  enters  into 
the  composition  of  the  woody  tissue.  The  oxygen  liber- 
ated by  vegetation  supports  animal  life,  while  the  vege- 
table substance  built  up  from  inorganic  compounds  either 
dies  or  is  used  as  food  for  animals,  and  so  in  turn  becomes 
part  of  animal  tissue. 

Dead  vegetable  and  animal  matter,  together  with  the 
matter  excreted  by  animals  during  life  marks  another  stage 
in  the  cycle  of  changes.  Dead  organic  matter  cannot  be 
used  as  food  by  higher  forms  of  vegetation.  It  contains 
all  of  the  material  necessary  to  vegetable  life,  but  not  in 
suitable  form.  The  organic  matter  must  first  be  worked 
over  by  lower  forms  of  life,  which  liquefy  the  solids,  release 
the  carbon  dioxide  and  convert  the  nitrogenous  material 
into  nitrates.  This  function  is  performed  by  fungi,  yeasts, 


2  SEWAGE    PURIFICATION    AND    DISPOSAL 

bacteria  and  moulds,  which,  having  no  chlorophyl  with 
which  to  decompose  from  the  carbon  dioxide  of  the  air  the 
cellulose  forming  carbon,  derive  the  cellulose  which  they 
contain,  as  well  as  all  the  substances  by  which  they  are 
nourished,  from  organic  matter,  either  living  or  dead. 
They  subsist  like  animals,  by  devouring  plants  or  other 
animals;  not  like  higher  plants,  which  derive  their  nutri- 
ment from  the  soil  and  air.  When  this  class  of  organisms 
have  completed  their  work,  dead  organic  matter  has  been 
reduced  to  inorganic  compounds  suitable  for  plant  life,  and 
the  cycle  of  change  is  complete. 

That  is  the  process  going  on  in  nature  at  all  times, 
while  in  the  purification  of  sewage  the  last  stage  of  the 
process,  that  which  reduces  the  organic  to  inorganic,  is 
carried  on  under  artificial  conditions  that  are  the  most  fav- 
orable for  the  purifying  organisms. 

A  sewage  purification  plant  is  primarily  a  bacterial 
farm,  and  to  successfully  design  and  operate  one  requires 
a  general  knowledge  of  the  composition  and  decomposition 
of  sewage,  together  with  a  thorough  knowledge  of  the 
kind,  functions  and  habits  or  requirements  of  the  invisible 
vegetation  to  be  cultivated.  It  is  upon  their  multiplica- 
tion, species  and  activity  that  the  success  of  the  plant 
depends,  and  to  promote  such  growth,  natural  selection 
and  activity,  the  conditions  of  light,  air,  food,  temperature, 
dilution  and  space  they  require,  under  different  conditions, 
must  be  known  and  provision  made  for  these  requirements. 

Organic  flatter — In  its  broadest  sense,  organic  matter 
is  anything  that  possesses  life,  or  is  the  product  of  the 
vital  activity  of  anything  that  possesses  life.  It  might  be 
vegetable  matter  like  potatoes,  rice  or  flax ;  animal  matter 
like  flesh,  blood  and  bones;  the  product -of  vegetable  life 
like  the  sap  of  a  maple  tree,  or  the  product  of  the  vital 
activity  of  animals  in  the  form  of  milk,  urine  and  excreta. 
Organic  matter  may  be  either  living  or  dead.  Life  is  intra- 
cellular  activity;  death,  cessation  of  activity.  In  the 
living  condition,  organic  matter  is  active.  It  possesses 
intra-cellular  motion,  together  with  certain  vital  resistance, 


SEWAGE    PURIFICATION    AND    DISPOSAL  3 

that  protects  it  from  injury  from  without.  After  death 
both  the  motion  and  the  resistance  cease,  and  the  cells 
that  compose  the  organic  matter  become  food  for  lower 
organisms,  which  reduce  them  to  useful  compounds. 

In  the  fluid  waste  that  makes  up  a  town's  sewage, 
organic  matter  is  present  in  considerable  quantity,  both 
as  vegetable  matter  and  as  animal  matter,  living  and  dead. 
It  is  only  the  dead  organic  matter,  however,  that  gives  to 
sewage  its  characteristic  appearance  and  odor.  This  mat- 
ter is  very  unstable  and  is  constantly  undergoing  changes 
that  break  it  down  into  simple  compounds,  usually  with  an 
evolution  of  foul-smelling  gases.  As  a  rule  the  quantity 
of  .organic  matter  in  average  sewage  does  not  exceed  one 
part  in  1,000,  yet  it  is  this  small  portion  of  the  whole  that 
makes  sewage  objectionable,  and  it  is  the  object  of  sewage 
purification  works  to  oxidize  into  useful  nitrate  that  one 
part  of  organic  matter. 

It  is  commonly  supposed  that  urine  and  excreta  are 
the  two  matters  that  make  sewage  objectionable.  As  a 
matter  of  fact,  however,  they  contribute  only  their 
portion  to  the  general  result.  If  the  discharges  from 
water  closets  and  urinals  were  excluded  from  the  house 
drains,  the  composition  of  the  sewage  would  be  so  little 
altered  that,  to  the  senses,  the  change  would  not  be  per- 
ceptible. However,  while  urine  and  excreta  add  but  little 
to  the  objectionable  appearance  and  odor  of  sewage,  they 
are  objectionable  for  sanitary  reasons,  as  under  certain 
conditions  they  might  infect  a  water  supply.  For  instance, 
when  an  individual  suffers  from  a  bacterial  disease  of  the 
bowels,  such  as  typhoid  fever  or  cholera,  his  discharges 
contain  the  specific  bacteria  of  that  disease  and  might 
infect  the  sewage  and  thence  find  its  way  to  some  water 
supply,  thereby  causing  an  epidemic  of  the  disease. 

The  organic  matter  of  household  sewage,  outside  of 
excreta  and  urine,  consists  of  tea  and  tea  leaves;  coffee 
and  coffee  grounds;  soap  suds  from  washing,  scrubbing, 
laving  and  scouring;  finely  divided  organic  matter  removed 
with  soap  suds ;  grease  from  cooking  and  from  the  cleaning 


4  SEWAGE    PURIFICATION    AND    DISPOSAL 

of  cooking  utensils;  crumbs  of  bread  and  portions  of  other 
foods  carried  into  the  drain  during  the  process  of  wash- 
ing dishes;  paper  from  water  closets;  milk,  juices  of  fruits 
and  vegetables  extracted  during  the  process  of  cleaning, 
preparing  and  cooking,  and  numerous  other  matters  that 
enter  into  the  composition  of  the  household  wastes. 

The  sewage  from  manufacturing  towns  is  more  com- 
plex than  that  from  resident  cities.  In  the  former  case 
many  by-products  of  the  various  industries  are  added  to 
the  usual  household  waste.  These  by-products  so  change 
the  character  of  the  sewage  that,  to  a  greater  or  less 
extent,  to  produce  satisfactory  results  it  requires  modifica- 
tions in  the  construction  and  operation  of  the  sewage 
purification  works.  Some  of  the  industrial  wastes  that 
must  be  cared  for  by  purification  plants  are:  blood,  hair, 
portions  of  animal  tissue  and  the  contents  of  animal  intes- 
tines discharged  into  sewers  by  slaughter  houses;  malt 
and  grain  from  breweries;  woody  fibers  from  paper  mills; 
hair,  wool,  cotton  and  felt  from  cotton  mills,  wool  scour- 
ing, felting  and  textile  works;  drainage  from  stables,  and 
portions  of  vegetable  matter  from  canning  and  preserving 
factories.  In  addition  to  the  foregoing  organic  waste, 
some  industrial  concerns  discharge  into  the  sewer  liquid 
wastes  that  are  so  strong  that  they  greatly  interfere  with 
the  ordinary  biological  process  of  decomposition.  Among 
such  liquid  wastes  may  be  mentioned  dyes  from  dyeing 
works;  chemicals  from  bleaching  works;  acids  from  tan- 
neries, and  galvanizing  pickle  from  galvanizing  plants. 

Sewage  purification  plants  in  cities  that  have  the 
combined  system  of  sewers,  that  is,  a  system  in  which  both 
rain  water  and  sewage  are  conveyed,  have  imposed  upon 
them  the  further  burden  of  purifying  the  washing  from 
street  surfaces,  yards,  roofs  and  areas.  This  in  itself  is  no 
inconsiderable  burden,  as  it  necessitates  the  reduction  of 
most  of  the  organic  matter,  such  as  droppings  from  horses, 
that  litter  the  streets  before  a  rain  storm. 

The  living  organic  matter  in  sewage  consists  chiefly  of 
the  micro-organisms  that  bring  about  the  change  in  the 


SEWAGE    PURIFICATION    AND    DISPOSAL  5 

composition  of  the  dead  organic  matter.  These  multiply 
rapidly  when  food  is  plentiful  and  the  conditions  of  air 
and  temperature  are  suitable,  but  they  die  quickly  when 
the  environment  becomes  unsuitable  for  their  wants,  and 
are  soon  reduced  to  inorganic  compounds  by  the  surviving 
organisms. 

Nitrogenous  flatter — Nitrogenous  matter  is  widely  dis- 
tributed in  nature,  and  in  some  form  or  other  enters  into 
the  composition  of  all  organic  matter,  both  vegetable  and 
animal.  The  source  of  all  nitrogenous  matter  is  the  atmo- 
sphere, where,  as  nitrogen,  it  composes  the  greater  bulk  of 
the  air.*  In  its  gaseous  state,  nitrogen  cannot  be  assimi- 
lated by  animal  and  vegetable  organisms.  For  the  use  of 
vegetation  nitrogen  must  first  be  reduced  to  soluble 
nitrates, f  while  animal  and  lower  forms  of  vegetable  life, 
like  fungi,  moulds  and  bacteria,  obtain  their  nitrogen  from 
the  proteids  and  albumin  built  up  by  chlorophyl  bearing 
plants  or  by  other  animals. 

During  the  time  that  nitrogen  is  a  constituent  of  organic 
life,  either  vegetable  or  animal,  it  is  a  stable,  insoluble  sub- 
stance; when  the  organic  matter  is  dead,  however,  the 
nitrogenous  substance  becomes  soluble  and  unstable,  and  is 
quickly  attacked  by  putrefactive  micro-organisms,  which 
reduce  it  to  more  simple  compounds.  Nitrogenous  com- 
pounds are  what  give  to  the  organic  matter  in  sewage  its 
most  objectionable  quality. 

The  principal  class  of  nitrogenous  compounds  entering 
into  the  composition  of  animal  and  vegetable  organism  is 
albumin.  This  is  such  a  common  constituent  of  organic 
matter  in  sewage  that  the  amount  found  by  chemical  anal- 
ysis is  taken  as  an  index  of  the  quantity  of  organic  matter 
in  the  sewage.  A  familiar  example  of  albuminoid  matter 
is  the  transparent  albumen  that  forms  what  is  known  as  the 


*Air,  by  volume;  .21  oxygen,  .78  nitrogen  and  .1  argon. 

t  Most  vegetation  obtain  their  nitrogen  from  organic  matter  that  has  been 
reduced  to  nitrates  by  nitrifying  bacteria.  Some  plants,  however,  particularly 
those  belonging  to  the  leguminous  group,  obtain  their  supply  of  nitrates  from 
nitrogen  fixing  bacteria  that  grow  in  nodules  on  the  roots  of  the  plants,  and 
obtain  their  nitrogen  direct  from  the  atmosphere. 


6  SEWAGE    PURIFICATION    AND    DISPOSAL 

white  of  eggs.  Urine,  also  rich  in  albumin,  is  another 
familiar  example.  In  vegetable  organisms  albumin  is 
present  in  the  fluids  and  solids  of  the  plant,  while  the  fruit 
of  leguminous  plants,  like  beans,  lentils  and  peas,  also  the 
meat  of  nuts,  are  rich  in  nitrogenous  materials.  The  dif- 
ferent forms  of  nitrogenous  materials — albumin,  protein, 
globulin,  etc. — are  all  composed  of  the  same  substance,  but 
contain  varying  proportions  of  the  elements — carbon, 
hydrogen,  oxygen,  nitrogen  and  sulphur — of  which  all 
nitrogenous  compounds  are  formed. 

Carbohydrates — Vegetable  organisms,  composed  of  car- 
bon, hydrogen  and  oxygen,  in  any  of  the  various  propor- 
tions that  these  gases  may  be  grouped  to  form  stable  com- 
pounds, are  known  as  carbohydrates.  One  of  the  chief 
constituents  of  carbohydrates  is  starch,  or  its  derivative, 
sugar.  Familiar  examples  of  carbohydrates  are  sugar- 
cane, glucose  and  grapes. 

Carbohydrates  are  always  of  vegetable  origin.  Their 
presence,  therefore,  is  not  indicative  of  animal  pollution, 
while  nitrogenous  substances  might  be  either  of  vegetable 
or  of  animal  origin. 

Cellulose — Cellulose  is  a  carbohydrate,  to  the  usual 
molecules  of  which  is  added  one  atom  of  nitrogen.  Cellu- 
lose is  interesting  in  sewage  purification,  principally  from 
the  fact  that  it  is  the  chief  constituent  in  the  composition 
of  the  woody  fiber  that  makes  up  the  stalk,  branches  and 
leaves  of  plants.  Filter  paper,  bleached  cotton  and  cocoa- 
nut  fiber  are  nearly  pure  cellulose. 

Inorganic  Matter — The  inorganic  matter  in  sewage 
usually  is  in  solution,  and  consists  principally  of  mineral 
salts  that  have  been  dissolved  by  the  water  when  perco- 
lating through  the  earth  or  running  along  its  surface. 
Sometimes  inorganic  matter  is  in  suspension  in  finely 
divided  particles  like  the  minute  flakes  of  clay  that  cause 
the  turbidity  of  some  rivers.  Most  of  the  inorganic  matter 
in  sewage  is  present  as  a  constituent  of  the  water  that 
supplies  the  community,  or  that  has  leaked  into  the  sewer 
from  the  ground  water.  The  most  common  forms  of 


SEWAGE    PURIFICATION    AND    DISPOSAL  7 

inorganic  matter  in  solution  in  sewage  are  acid  wastes  from 
factories,  carbonates  of  lime,  sulphates  of  lime,  ferrous 
oxide,  salt,  in  the  form  of  chlorides,  chiefly  of  sodium,  with 
less  quantities  of  potassium  and  ammonium,  and  nitrates. 
In  suspension  there  is  present  more  or  less  sand,  grit, 
clay,  silt  and  coarse  particles  in  the  form  of  pins,  hair- 
pins and  other  metallic  substances  that  have  been  acci- 
dentally introduced  into  the  drainage  system. 

The  process  of  sewage  purification  is  less  affected  by 
the  presence  of  coarse  metallic  particles  than  by  the  matter 
in  solution.  Inorganic  matter  in  suspension  can  easily  be 
removed  by  precipitation  in  a  grit  chamber,  or  it  can  be 
caught  on  the  rough  screen  that  usually  protects  the  raw 
sewage  inlet  to  purification  works.  If  it  passes  the  grit 
chamber  and  screen,  upon  entering  the  tank  it  settles  in 
the  sludge  at  the  bottom  and  causes  no  trouble.  Soluble 
impurities  on  the  other  hand,  affect  the  character  of  the 
sewage  and  modify  its  chemical  composition  to  such  an 
extent  that  different  treatment  is  required  for  sewage  con- 
taining different  inorganic  impurities.  For  instance,  car- 
bonates of  lime  cause  the  property  in  water  known  as 
temporary  hardness.  Nitrogenous  matter  in  advanced 
stages  of  decomposition  is  present  as  nitric  acid.  To  form 
nitrates,  and  thus  complete  the  process  of  purification,  the 
nitric  acid  must  have  a  suitable  alkaline  base  to  react 
upon.  Lime  in  the  form  of  carbonates  is  a  suitable  base 
that  readily  combines  with  nitric  acid  and  forms  nitrate 
of  lime. 

It  follows,  therefore,  that  sewage  which  is  moderately 
alkaline  in  reaction  is  the  most  suitable  for  purification, 
while  sewage  composed  of  a  soft  water,  like  many  water 
supplies  from  surface  sources,  must  be  made  moderately 
temporarily  hard  before  nitrification  can  take  place.  In 
practice,  soft  waters  are  made  sufficiently  hard  for  nitrifi- 
cation by  adding  a  suitable  amount  of  lime  to  the  sewage. 
Other  bases  can  be  used  for  this  purpose,  but  lime  is  gen- 
erally selected  for  economic  reasons  and  because  it  is 
readily  obtainable  in  any  market. 


8  SEWAGE    PURIFICATION    AND    DISPOSAL 

Water  containing  about  10  degrees  of  hardness,  accord- 
ing to  the  Clark- Wanklin  scale,  which  is  equal  to  14  parts 
or  grains  of  carbonate  of  lime  to  100,000  parts  of  water, 
would  probably  be  the  best  suited  for  ordinary  sewage. 
Where  the  sewage  receives  the  acid  waste  from  factories, 
however,  the  degree  of  hardness  required  to  neutralize  the 
acid  so  as  not  to-  interfere  with  the  ordinary  process  of 
decomposition  would  depend  upon  the  amount  of  acid  in 
the  sewage.  The  permissible  degree  of  hardness  in  a 
water  supply  is  20  degrees  Clark-Wanklin,  or  28  parts 
per  100,000,  and  if  this  amount  is  not  sufficient  to  neutral- 
ize the  acid  in  the  sewage,  the  deficiency  must  be  supplied 
by  adding  lime,  soda  or  some  other  base  to  the  sewage. 
Care  should  be  exercised,  however,  not  to  add  too  much 
lime,  as  an  excess  of  what  is  required  to  neutralize  the 
acid  will  precipitate  the  suspended  solids  and  part  of  the 
matter  in  solution,  besides  temporarily  inhibiting  the 
growth  and  activity  of  the  putrefactive  bacteria. 

Sewage  containing  inorganic  matter  in  the  form  of 
sulphate  of  lime,  when  undergoing  decomposition,  is  liable 
to  produce  sulphuretted  hydrogen  from  the  reduction  of 
sulphates,  and  the  effluent  from  purification  works  con- 
taining sulphate  of  lime,  particularly  when  iron  is  present 
in  solution  in  the  water,  is  liable  to  be  of  a  dark  color, 
while  the  lime  and  iron  together  in  the  tanks  is  liable  to 
act  as  a  precipitate  and  throw  down  part  of  the  suspended 
matter  in  the  sewage. 

Salt  in  the  form  of  chlorine  is  an  inorganic  constituent 
of  all  sewage.  It  is  present  in  varying  quantities  in  dif- 
ferent sewage,  but  does  not  seem  to  exert  an  unfavorable 
influence  on  its  decomposition.  In  fact,  there  is  reason  to 
believe  that  a  certain  quantity  of  chlorine  is  necessary  for 
the  exercise  of  bacterial  activity. 

Suspended  Matter — Particles  of  matter  in  sewage  that 
are  visible  without  the  aid  of  a  microscope  or  magnifying 
glass,  and  that  are  floating  in  the  liquid  either  on  the  sur- 
face or  submerged,  are  known  as  suspended  matter.  All 
matter  in  suspension,  except  the  fine  particles  of  clay  that 


SEWAGE    PURIFICATION    AND    DISPOSAL  9 

impart  turbidity  to  waters,  are  of  organic  origin.  Cloth, 
paper,  orange  peels,  etc.,  are  familiar  examples  of  matter 
carried  in  suspension  by  sewage.  Coarse  particles  of  sus- 
pended matter  are  found  principally  in  fresh  sewage. 
Before  reaching  a  sewer  outfall  or  purification  works,  most 
of  the  suspended  matter  in  sewage  is  in  a  finely  dissemi- 
nated state,  but  still  visible  to  the  naked  eye  and  still  in 
suspension.  The  disintegration  of  the  coarse  particles  of 
matter  is  brought  about  by  mechanical  means,  such  as 
scraping  along  the  walls  of  the  sewers,  and  by  a  biological 
process  due  to  micro-organisms. 

Colloidal  Matter — That  portion  of  the  suspended  matter 
of  sewage  or  sewage  effluents  that  is  in  emulsion  is  known 
as  colloidal  matter.  Such  matter  is  of  organic  origin, 
belongs  to  the  albuminoids  and  is  invisible.  It  marks 
one  of  the  stages  in  the  decomposition  of  some  albumi- 
noids, and  its  presence  in  the  effluent  from  sewage  purifi- 
cation works  indicates  that  the  reduction  of  albuminoids  is 
incomplete. 

Effluents  that  contain  colloidal  matter  are  liable  under 
certain  conditions  to  give  rise  to  subsequent  putrefaction. 
To  avoid  this,  it  is  fully  as  important  to  reduce  the  col- 
loidal matter  as  it  is  to  reduce  the  suspended  matter.  The 
amount  of  colloidal  matter  in  an  effluent  can  be  determined 
by  gathering  it  on  parchment.  It  is  a  jelly  or  glue-like 
substance  that  very  slowly  penetrates  parchment,  which 
being  pervious  to  water  permits  it  to  pass  but  holds  back 
the  colloidal  matter. 

Matter  in  Solution — Most  of  the  matter  in  solution  in 
sewage  is  inorganic,  and  consists  of  dissolved  salts  of  iron, 
lime,  chlorine  and  other  mineral  substances.  Chlorine  is 
usually  present  in  the  greatest  quantity,  and  the  amount 
remains  almost  constant  throughout  the  process  of  purifi- 
cation. About  40  per  cent,  of  the  matter  in  solution  in 
sewage  is  supposed  to  be  organic.  This  portion  of  organic 
matter,  however,  may  be  more  properly  classed  as  col- 
loidal, for  the  greater  portion  of  it  is  in  emulsion  not  in 
solution. 


10  SEWAGE    PURIFICATION    AND    DISPOSAL 

About  80  per  cent,  of  the  soluble  or  colloidal  organic 
matter  in  sewage  can  be  and  usually  is  removed  or  reduced 
to  harmless  compounds  by  the  process  of  purification. 
Very  little  of  the  soluble  inorganic  matter,  however,  is 
changed,  and  as  this  portion  of  the  sewage  is  non-putres- 
cible  and  non-injurious  to  health,  its  reduction  is  not  nec- 
essary. The  inorganic  matter  in  solution  in  sewage  is  of 
interest  only  when  from  its  chemical  composition  it  either 
aids  or  retards  the  natural  process  of  putrefaction  and 
nitrification. 

Detritus — That  portion  of  inorganic  matter  like  sand, 
gravel,  pebbles,  etc. ,  that  is  washed  into  the  street  sewers 
during  rain  storms  is  known  as  detritus.  Detritus  is  of 
little  importance  in  sewage  purification,  as  it  is  non-pu- 
trescible,  and  the  small  amount  that  passes  the  catch  basins 
in  the  rain-water  inlets  settles  to  the  bottom  of  the  septic  or 
settling  tank,  where  it  simply  adds  its  volume  to  the  sludge. 

Gas — Sewage  is  an  extremely  unstable  compound  that 
is  constantly  undergoing  changes  that  reduce  the  organic 
matter  to  simpler  forms.  The  reduction  of  organic  matter 
in  sewage  is  accompanied  by  an  evolution  of  gases,  some 
of  which  remain  in  solution  while  the  greater  portion  in 
free  state  escape  to  the  atmosphere.  The  gases  from 
decomposing  sewage  are  usually  inflammable,,  and  when 
collected  and  conducted  to  a  suitable  burner,  give  off  light 
and  heat  almost  as  intense  as  that  from  ordinary  illumi- 
nating gas.  In  tropical  countries,  where  the  bacterial 
activity  is  greatest  and  a  corresponding  amount  of  gas 
evolved,  the  gas  can  be  and  has  been  collected  in  a  gas- 
ometer and  used  for  light,  fuel  and  power  about  the  plant. 
The  gas  evolved  by  decomposing  organic  matter  generally 
consists  of  the  following  constituents: 

Carbon  dioxide 5.90 

Oxygen 76 

Methane  (marsh  gas) 75.18 

Nitrogen 17.40 

Hydrogen 26 

Undetermined       .  50 

Total  100 


SEWAGE    PURIFICATION    AND    DISPOSAL  11 

The  quantity  of  gas  given  off  by  sewage  depends 
greatly  upon  the  temperature,  about  twice  the  quantity 
being  evolved  during  the  summer  months  as^that  given  off 
during  cold  weather.  There  are  two  reasons  for  this.  In 
the  first  place,  the  optimum  temperature  for  bacteria,  at 
which  they  are  most  active,  is  that  of  summer  heat;  conse- 
quently, the  greater  activity  of  the  bacteria  during  warm 
months  would  result  in  the  evolution  of  a  greater  quantity 
of  gas.  In  the  second  place,  the  capacity  of  sewage  to 
absorb  gas  is  in  direct  proportion  to  its  temperature, 
being  least  during  warm  weather;  consequently  a  greater 
proportion  of  gas  would  be  held  in  solution  during  cold 
weather  and  would  pass  off  in  the  effluent. 

As  the  quantity  of  gas  given  off  by  decomposing 
organic  matter  is  an  index  of  the  rate  of  putrefaction,  the 
reduction  during  winter  months  indicates  clearly  that  in 
order  to  obtain  the  best  results,  means  should  be  provided 
at  purification  works  in  cold  climates  to  protect  the  sewage 
and  effluent  from  the  weather. 

The  quantity  of  gas  that  under  favorable  conditions 
is  given  off  from  decomposing  sewage  varies  with  the 
strength  and  composition  of  the  sewage,  and  ranges  from 
3  to  &  gallons  of  gas  during  twenty-four  hours  for  each  100 
gallons  of  sewage.  A  fair  average  would  probably  be  4 
gallons  or  about  y?  cubic  foot  of  gas  per  day  for  each  100 
gallons  of  sewage. 

Physical  Characteristics  of  Sewage — When  viewed  as 
it  leaves  the  sewer  at  an  outfall  or  at  a  purification  works, 
sewage  presents  the  appearance  of  slop  water  with  some 
solid  matter  in  suspension.  The  appearance  of  sewage 
varies  considerably  with  its  strength  and  staleness;  thus, 
strong,  fresh  sewage  would  contain  more  matter  in  suspen- 
sion than  would  a  strong,  stale  sewage,  while  strong  sewage 
generally  has  a  more  milky  appearance  than  a  weaker 
mixture.  The  strength  of  sewage  depends  on  the  propor- 
tions of  organic  matter  it  contains,  and  the  degree  of 
staleness  depends  on  the  length  of  time  it  has  been  exposed 
to  putrefactive  processes.  Both  of  these  characteristics 


12  SEWAGE    PURIFICATION    AND    DISPOSAL 

are  of  importance  in  sewage  purification.  The  strength  of 
sewage  determines  the  amount  that  can  be  successfully 
treated  by  filtration  in  a  given  time,  and  the  staleness  of 
sewage  determines  the  length  of  time  it  should  be  sub- 
jected to  septic  processes. 

The  condition  of  sewage  best  suited  for  purification  de- 
pends upon  the  method  employed.  If  sewage  is  to  be  treated 
at  a  precipitation  works  the  sewage  should  be  delivered  at 
the  plant  in  as  fresh  a  state  as  is  possible.  For  septic  treat- 
ment, on  the  other  hand,  the  more  stale  the  sewage  the  less 
time  it  requires  for  bacterial  action  at  the  purification  works. 

Strength  of  Sewage — Sewage  of  average  strength  con- 
tains, in  addition  to  the  organic  and  inorganic  matter  in 
the  water  supply,  about  one  part  of  organic  matter  and 
one  part  of  inorganic  matter  in  each  1,000  parts  of  sewage. 
In  a  well  designed  and  operated  purification  plant,  from  80 
to  90  per  cent,  of  the  organic  matter,  and  all  the  inorganic 
matter  in  solution,  can  be  removed ;  this  leaves  in  the  tanks 
to  form  sludge  only  that  portion  of  the  inorganic  matter 
that  is  in  suspension,  and  which  seldom  exceeds  .05  part  in 
1,000  parts  of  sewage. 

Sludge — Sludge  is  the  insoluble  portion  of  sewage  that 
cannot  be  liquefied,  mineralized  or  converted  into  gases. 
The  composition  of  sludge  depends  considerably  upon  the 
composition  of  the  sewage  and  whether  storm  water  enters 
the  sewers.  It  consists  mostly  of  mineral  matter  together 
with  some  organic  matter  and  about  90  per  cent,  of  water. 
Sludge  is  of  interest  chiefly  for  the  reason  that  the  residue 
that  cannot  be  liquefied  must  be  removed  from  the  tanks 
by  mechanical  means. 

Effluents — An  effluent  is  the  liquid  portion  of  sewage 
from  a  purification  plant  after  the  organic  matter  has  been 
reduced.  In  practice,  the  term  effluent  is  applied  to  the 
outflow  from  any  portion  of  a  purification  plant,  such  as 
from  a  septic  tank  to  filter  beds.  The  term  influent  refers 
to  the  treated  fluid  when  flowing  into  or  on  one  portion  of  a 
purification  plant  from  another  portion.  For  instance,  an 
effluent  from  a  septic  tank  would  be  an  influent  to  filter  beds. 


SEWAGE    PURIFICATION    AND    DISPOSAL  13 

Sewage  before  undergoing  the  process  of  purification 
is  sometimes  spoken  of  as  raw,  or  crude  sewage,  but  gen- 
erally it  is  known  simply  as  sewage. 

Variation  in  the  Flow  of  Sewage — Sewage  varies  from 
day  to  day  and  from  hour  to  hour,  not  only  in  volume  but 
also  in  its  composition.  The  variation  in  sewage  is  more 
marked  in  manufacturing  than  in  residence  towns,  while  at 
the  same  time  the  sewage  of  no  two  places  is  alike,  even 
though  in  each  place  it  is  of  domestic  character.  It  is  for 
this  reason  that  the  purification  and  disposal  of  sewage 
from  each  town  requires  special  study  and  treatment,  and 
rules  cannot  be  laid  down  that  are  applicable  to  all  cases. 

The  volume  of  sewage  to  be  cared  for  during  rain 
storms  is  much  greater  than  the  dry  weather  flow,  and  this 
increase  in  volume  usually  continues  for  some  time  after  a 
storm,  during  which  time  ground  water  finds  its  way  into 
the  sewers.  The  increase  in  volume  of  sewage  due  to  storm 
water  is  generally  compensated  for  by  the  correspondingly 
weaker  sewage  to  be  treated,  which  permits  a  greater  quan- 
tity to  be  purified  in  a  given  time  on  a  certain  area  of 
filter  surface. 

The  dry  weather  flow  of  sewage  varies  greatly  both 
in  strength  and  in  volume  during  the  twenty-four  hours 
of  a  day,  being  greater  in  the  day  time,  when  the  waste 
from  factories  is  added  to  the  domestic  sewage.  The 
strength  and  volume  of  sewage  fluctuates  from  hour  to 
hour  during  the  day  in  residence  towns,  being  greatest 
during  those  hours  occupied  in  the  preparation  of  meals, 
while  in  manufacturing  towns  great  vats  of  manufacturing 
fluids  might  be  discharged  into  the  sewers  at  any  time,  thus 
changing  in  a  few  minutes  both  the  quantity  and  chemi- 
cal composition  of  the  sewage.  During  the  night,  when 
the  flow  of  sewage  is  light,  it  is  also  correspondingly  weak, 
and  is  made  up,  to  a  large  extent,  of  pure  water  leaking  in 
at  plumbing  fixtures  and  ground  water  which  bears  a  greater 
proportion  to  the  night  than  to  the  day  flow  of  sewage. 

The  variation  in  volume  and  composition  of  sewage  has 
a  marked  influence  on  the  construction  and  operation  of  a 


14  SEWAGE    PURIFICATION    AND    DISPOSAL 

purification  plant,  the  design  of  which  should  not  be 
undertaken  until  data  covering  such  subjects  are  at  hand. 
Means  can  then  be  devised  to  regulate  the  flow  and  bring 
about  a  more  uniform  mixture  of  the  composition. 

Temperature  of  Sewage— The  temperature  of  sewage 
is  an  important  consideration  in  the  process  of  purification. 
The  micro-organisms  that  liquefy  organic  matter  flourish 
best  in  a  medium  with  but  slight  range  of  temperature, 
which  does  not  drop  below  50  degrees  Fahr.  Below  that 
temperature  little  bacterial  activity  takes  place  and  con- 
sequently liquefaction  of  solids  is  at  a  minimum.  In  cold 
weather  the  temperature  of  sewage  varies  with  and  is 
generally  about  10  degrees  warmer  than  the  temperature 
of  the  atmosphere.  During  the  summer  months,  the  tem- 
perature of  sewage  averages  about  75  degrees  Fahr.,  and 
seldom  rises  above  80  degrees  Fahr.  Summer  temperature 
of  sewage  is  the  most  favorable  for  the  liquefaction  of 
solids.  This  is  evidenced  by  the  fact  that  organic  matter 
accumulates  in  septic  tanks  during  winter  weather  and  is 
destroyed  when  the  temperature  rises  in  the  spring  and 
summer.  In  designing  a  sewage  purification  plant,  the 
climate  where  it  is  to  be  constructed  should  be  taken  into 
consideration,  and  if  the  mean  temperature  for  December, 
January  and  February  is  below  40  degrees  Fahr. ,  to  secure 
satisfactory  liquefaction  of  the  solids,  all  tanks,  reservoirs 
or  wells  that  contain  much  sewage  should  be  enclosed 
from  the  weather.  Filter  beds,  however,  that  are  suitably 
underdrained  need  not  be  protected  from  the  weather. 
Most  of  the  sewage  rapidly  drains  out  of  the  sand,  leaving 
the  interstices  filled  with  air  and  what  liquid  remains  is 
quickly  thawed  when  sewage  is  again  applied  to  the  bed. 


DECOMPOSITION  OF  5EWAQE 

Bacteria — In  nature,  organic  matter  is  broken  down 
into  inorganic  compounds  by  protozoa,  fungi,  yeast,  moulds 
and  bacteria.  In  the  artificial  environment  created  by  a 
sewage  purification  works,  however,  bacteria  play  the  most 


SEWAGE    PURIFICATION    AND    DISPOSAL  15 

important  part  in  the  reduction.  Bacteria  are  minute 
organisms  belonging"  to  the  vegetable  kingdom,  and  closely 
allied  to  algae,  fungi,  yeasts  and  moulds.  They  possess  n«  • 
chlorophyl  or  green  parts  to  decompose  carbon  dioxide 
from  the  air,  so  they  live  more  like  animals  than  like 
plants,  an'd  secure  their  food  by  wresting  their  cellulose,  as 
well  as  the  other  constituents  entering  into  their  compo- 
sition, from  animal  or  vegetable  tissue,  either  living  or 
dead.  Bacteria  consist  of  but  one  cell,  which  in  different 
species  assumes  different  shapes.  Some  known  as  bacilli 
are  slightly  curved  rods  with  a  length  of  two  or  more  diam- 
eters. Micro-coccos  are  little  spherical  shaped  bacteria  that 
perhaps  are  the  most  minute  of  all  forms,  while  the  spiril- 
lum are  shaped  like  the  coil  of  a  corkscrew  or  the  spiral  of 
a  spring.  Bacteria  are  very  prolific,  and  make  up  in  the 
rate  of  reproduction  what  they  lack  in  size.  With  plenty 
of  food  and  in  suitable  environment,  a  single  bacterium 
in  twenty-four  hours  will  produce  16,000,000  progeny;  in 
two  days  24,000,000,000,  and  in  a  week  the  number  express- 
ing their  quantity  would  be  made  up  of  fifty-nine  figures. 
As  a  matter  of  fact,  however,  conditions  are  never  suit- 
able for  this  enormous  growth.  Each  specie  of  bacteria 
requires  certain  substances  to  feed  upon,  and  when  that 
particular  substance  is  exhausted,  or  nearly  so,  the  bacteria 
must  strive  with  the  more  fortunate  species,  existing  in 
what  to  them  is  a  favorable  environment;  and,  in  the 
struggle  that  ensues,  the  doctrine  of  the  survival  of  the 
fittest  holds  true  and  one  specie  of  bacteria  ceases  to  exist 
that  the  others  may  live.  Even  in  mediums  where  the 
conditions  of  food,  temperature,  and  moisture  are  favor- 
able, other  conditions  interfere  to  inhibit  or  retard  the 
growth  of  bacteria.  This,  perhaps,  is  nature's  way  of 
holding  in  check  the  growth  of  micro-organisms,  which, 
left  unchecked,  might  become  more  of  a  menace  than  a 
blessing  to  the  world.  Each  bacterium  in  the  exercise  of 
its  natural  functions,  excretes  a  substance  that  dissolves  the 
material  it  requires  for  food,  thus  liberating,  in  assimilable 
form,  the  particular  portion  required  by  the  organism. 


16  SEWAGE    PURIFICATION    AND    DISPOSAL 

The  solvents  elaborated  by  bacteria,  when  attacking  dead 
organic  matter,  are  known  as  enzymas;  when  producing 
disturbance  in  a  living  host,  they  are  known  as  toxins 
(poisons).  These  toxins  or  enzymas,  when  they  exceed  a 
certain  percentage  of  the  medium  in  which  the  bacteria 
exist,  act  as  a  poison  to  the  bacteria  that  elaborate  them 
and  inhibit  their  further  growth.  They  do  not,  however, 
interfere  with  the  activity  of  other  species  of  bacteria 
which  can  take  up  the  work  at  this  point  and  carry  it  a  step 
further  towards  its  final  reduction.  A  familiar  illustration 
of  this  phenomenon  can  be  cited  in  the  case  of  alcoholic 
fermentation  of  grape  juice.  When  the  alcohol  elaborated 
by  the  alcoholic  ferments  (yeast)  exceeds  19  per  cent.,  the 
alcoholic  ferments  disappear  and  the  grape  juice,  in  which 
the  sugar  has  become  converted  into  alcohol,  has  become 
wine.  If  sterilized  and  protected  from  air,  the  liquid  will 
remain  in  this  condition  indefinitely,  even  though  sufficient 
food  remains  to  support  abundant  bacterial  life ;  should  the 
wine  be  left  exposed  to  the  atmosphere,  however,  a  new 
specie  of  yeast,  the  lactic  ferment,  would  take  possession 
and  convert  the  wine  into  vinegar. 

So  long  as  the  enzymas  or  toxine  of  bacteria  can  be 
kept  below  a  certain  percentage,  the  organisms  will  con- 
tinue to  exercise  their  function  of  breaking  down  organic 
matter.  The  simplest  and  most  practical  way  to  keep 
down  the  percentage  of  enzymas  is  to  remove  the  products 
as  soon  as  they  are  formed.  In  sewage  purification,  by  the 
septic  process,  this  is  accomplished  by  passing  a  continuous 
flow  of  sewage  through  the  tanks,  thus  providing  an  ade- 
quate supply  of  food  for  the  bacteria,  and  at  the  same 
time  removing  the  products  of  their  activity. 

Bacteria  multiply  by  fission  and  reproduce  by  spores. 
In  the  process  of  multiplication,  each  bacterium  grows  to 
about  twice  its  original  size,  gradually  constricting  at  the 
center  until  both  parts  have  attained  their  full  growth, 
when  they  part  at  the  constriction  and  each  half  starts 
on  an  independent  existence. 

When  thrust  into  a  favorable  environment,  bacteria 


SEWAGE    PURIFICATION    AND    DISPOSAL  17 

immediately  begin  to  vegetate,  and  so  long  as  food  is 
plentiful  in  an  easily  assimilable  form,  they  continue  to 
multiply  with  the  production  of  but  little  of  the  solvent 
fluids  that  break  down  the  coarser  solids.  When,  how- 
ever, the  bacteria  have  multiplied  to  such  an  extent  that 
the  easily  assimilable  food  is  rapidly  becoming  exhausted, 
or  when  for  any  other  reason  the  medium  becomes  an 
unfavorable  one,  the  energy  of  the  organisms  is  required 
to  secrete  and  elaborate  solvent  substances  to  prepare  food 
for  their  requirements,  and  the  period  of  multiplication 
comes  to  an  end.  That  is  the  process  which  takes  place 
in  the  septic  tank,  contact  bed,  or  intermittent  filter,  when 
sewage  is  first  applied,  or  when  the  tank  or  bed  is  put  in 
service  after  a  long  period  of  rest.  The  sewage  is  seeded 
with  bacteria  from  the  air,  water  and  soil,  and  a  period  of 
vegetation  ensues  during  which  the  organisms  best  suited 
for  the  purpose  multiply  to  such  an  extent  that  the  readily 
obtainable  supply  of  food  becomes  exhausted.  During  this 
period  very  little  reduction  of  the  organic  matter  takes 
place,  and  the  effluent  is  but  slightly  purified.  Following 
the  vegetative  stage  comes  a  period  of  bacterial  activity  in 
the  bed  or  tank  which  is  then  said  to  be  ripe,  and  the 
reduction  of  organic  matter  and  consequent  purification 
of  the  sewage  is  at  its  maximum. 

1  When  a  medium  becomes  so  unfavorable  for  a  particu- 
lar bacterium  that  it  can  no  longer  survive,  its  entire  vital 
energy  is  directed  towards  forming  a  little  spore,  or  seed, 
which  possesses  remarkable  vitality  and  can  withstand 
extremes  of  heat,  cold,  drought  or  moisture  that  would 
quickly  kill  the  mother  cell.  This  is  the  little  organism's 
way  of  perpetuating  its  species,  or  of  reproducing  itself. 
The  spore,  containing  dormant  vitality,  remains  unaffected 
by  the  surviving  organisms  and  continues  in  that  resting 
state  until  the  proper  conditions  of  heat,  moisture  and 
food  cause  it  to  germinate  and  set  up  a  living  ferment  or 
putrefaction. 

Classification   of   Bacteria — Bacteria  are  classified  ac- 
cording to  their  mode  of  living,  as  aerobics,  anaerobies  and 


18  SEWAGE    PURIFICATION    AND    DISPOSAL 

as  facultatives.  Aerobic  bacteria  live  only  in  the  presence 
of  free  air  or  oxygen ;  anaerobic  bacteria  thrive  only  in  the 
absence  of  air  or  oxygen,  while  facultative  bacteria  accustom 
themselves  either  to  the  presence  or  absence  of  free  air  or 
oxygen,  and  live  indifferently  either  as  aerobics  or  as 
anaerobies.  When  the  more  favored  habitat  of  facultative 
bacteria  is  in  the  presence  of  air  or  oxygen,  they  are  known 
as  facultative  aerobies.  When  they  thrive  better  in  the 
absence  of  free  air  or  oxygen  they  are  known  as  faculta- 
tive anaerobies.  Aerobic,  anaerobic  and  facultative  bacte- 
ria each  play  an  important  part  in  the  purification  of  sew- 
age. The  obligate  anaerobies,  that  is,  those  that  live  only 
in  the  absence  of  free  air  or  oxygen,  though  they  are  com- 
paratively few,  are  exceedingly  active  and  bring  about  the 
reduction  of  solid  matter  at  the  bottom  of  septic  tanks  and 
contact  beds. 

Facultative  anaerobies,  on  the  other  hand,  are  less 
active  than  the  obligate  anaerobies,  but  they  are  far  more 
numerous,  as  would  be  expected,  being  better  suited  to 
live  in  a  medium  that  contains  little  or  no  oxygen,  but 
may  at  any  time  become  aerated.  Obligate  aerobies  are 
present  in  the  first  and  last  stages  of  sewage  purification. 
In  the  first  stage,  they  prepare  the  way  for  the  more  active 
anaerobies  by  exhausting  the  supply  of  oxygen  in  the 
sewage.  In  the  last  stage  they  play  the  final  part  of 
reducing  the  nitrites  to  harmless  nitrates. 

It  should  not  be  inferred  that  because  some  bacteria 
live  only  in  the  absence  of  oxygen,  that  oxygen  is  not 
necessary  to  their  existence.  They  obtain  their  oxygen 
like  fish  and  aquatic  animals,  by  wresting  it  from  the 
medium*  in  which  they  exist,  or  from  organic  matter  con- 
tained in  that  medium,  and  are  destroyed  by  being  thrust 
into  an  atmosphere  of  air  or  oxygen.  For  instance, 
Pasteur  found  that  yeast,  which  he  classed  as  both  aerobic 
and  anaerobic,  when  introduced  to  a  sugar  solution,  con- 
taining no  oxygen,  obtained  the  oxygen  required  for  their 
existence  from  the  sugar  or  from  the  water  present. 

Bacteria  are   still  further  classified  according  to  the 


SEWAGE    PURIFICATION    AND    DISPOSAL  19 

food  they  require,  as  saprophytes  and  as  parasites.  Sapro- 
phytic  bacteria  live  on  dead  organic  matter,  either  animal 
or  vegetable,  or  on  the  secretions  or  excretions  of  plant  or 
animal  life.  It  is  the  saprophytic  bacteria  that  are  chiefly 
interesting  in  sewage  purification,  as  it  is  due  to  their 
activity  that  organic  matter  becomes  mineralized.  Para- 
sitic bacteria  can  live  and  thrive  only  within  a  living  organ- 
ism. They  can  exist  for  a  limited  time  outside  of  a  living 
host,  but  their  vitality  constantly  diminishes  in  the  unfav- 
orable environment,  until  at  last  they  cease  to  exist. 

It  is  quite  conceivable  that  a  third  class  of  bacteria 
might  exist  that  can  adapt  themselves  to  circumstances 
and  live  equally  well  as  parasites  or  as  saprophytes,  and 
that  under  some  conditions  such  bacteria  might  become 
pathogenic.  Parasitic  bacteria  that  produce  functional  dis- 
turbance in  an  organism,  once  they  gain  lodgment  therein, 
are  known  as  pathogenic.  All  parasitic  bacteria,  however, 
are  not  pathogenic ;  many  non-pathogenic  species  are  pres- 
ent during  life  in  the  warm  moist  lining  of  the  intestines, 
which,  containing  a  plentiful  supply  of  food,  provides  a 
favorable  environment  for  bacterial  life. 

Parasitic  bacteria  are  only  indirectly  interesting  in 
the  study  of  sewage  purification.  No  special  provision  is 
made  at  purification  works  to  destroy  the  parasitic  or  path- 
ogenic bacteria,  the  general  mineralization  of  the  organic 
matter  being  depended  upon  to  destroy  the  parasites. 

Saprophytic  or  septic  bacteria  may  be  distinguished 
as  putrefactive  bacteria,  that  is,  those  which  break  down 
nitrogenous  substances  into  ammonic  compounds,  carbon 
dioxide  and  water,  and  fermentative  bacteria  that  break 
down  carbohydrates  and  other  non-nitrogenous  substances 
into  carbon  dioxide,  water  and  methane  gas. 

Products  of  Bacteria — Under  favorable  conditions,  each 
species  of  bacteria  selects  for  its  nourishment  certain  suit- 
able substances  found  in  the  medium  where  it  exists,  and 
in  the  process  of  assimilation  excretes  a  substance  peculiar 
to  that  particular  species,  and  by  which  it  can  be  iden- 
tified. Certain  species  of  bacteria,  however,  not  only  are 


20  SEWAGE    PURIFICATION    AND    DISPOSAL 

able  to  avail  themselves  of  different  kinds  of  foodstuffs 
but  the  enzymas  they  excrete  differ  with  the  food.  For 
instance,  the  bacilli  of  cholera  when  growing  on  sugar, 
which  is  a  carbohydrate,  produce  butyric  acid,  the  sub- 
stance that  gives  butter  its  rancid  taste ;  but  when  growing 
on  nitrogenous  material,  excretes  a  toxalbumen  that  is 
poisonous. 

Fermentation  of  Carbohydrates — To  bring  about  a  fer- 
mentation of  carbohydrates,  some  nitrogenous  material, 
also  some  extratives  and  salts  such  as  are  found  in  the 
ashes  of  burnt  yeast,  must  be  present  in  the  medium.  This 
material  is  probably  what  the  micro-organisms  live  on 
while  producing  enzymas  that  convert  starch  into  sugar. 
In  the  alcoholic  fermentation  of  carbohydrates,  such  as 
grape  juice,  the  starch  must  be  converted  into  sugar  before 
a  true  fermentation  or  splitting  up  and  hydration  can 
begin.  The  products  of  carbohydratic  fermentation  are 
alcohol,  carbon  dioxide,  water,  succinic  acid  and  glycerine. 
True  carbohydrates  are  among  the  most  easily  reduced 
substances  found  in  sewage,  and  less  residue  is  left  after 
the  reduction  than  from  any  other  constituent.  The  alcohol 
and  carbon  dioxide  being  volatile,  escape  as  gases,  while 
the  succinic  acid  and  glycerine,  which  are  present  only  in 
very  small  quantities,  are  reduced  to  more  simple  com- 
pounds in  the  subsequent  process  of  purification  through 
which  they  pass. 

The  fermentation  of  carbohydrates  is  an  anaerobic 
process  carried  on  in  the  absence  of  air ;  consequently  the 
reduction  reaches  the  maximum  in  either  septic  tanks  or 
contact  beds,  where  the  supply  of  oxygen  is  exhausted. 
The  conditions  necessary  for  the  fermentation  of  carbohy- 
drates are  nitrogenous  material  in  a  soluble  condition,  phos- 
phoric acid,  and  carbohydrate  like  grape  sugar,  capable  of 
fermentation. 

Cellulose  Fermentation — Next  to  the  putrefaction  of 
nitrogenous  matter,  the  fermentation  of  cellulose  is  the 
most  important  reduction  in  sewage  purification.  Rags, 
paper,  vegetable  fiber,  and  other  like  material,  that  under 


SEWAGE    PURIFICATION    AND    DISPOSAL  21 

ordinary  conditions  are  stable  and  resist  the  reducing  influ- 
ences of  bacteria  for  a  long1  time,  when  placed  in  a  septic 
tank  are  quickly  broken  down  and  destroyed.  For 
instance,  in  some  experiments  conducted  by  the  Massachu- 
setts State  Board  of  Health,  at  Lawrence,  to  determine  the 
length  of  'time  required  to  break  down  cellulose  material,  a 
considerable  quantity  of  newspaper,  cotton  and  woolen 
cloth,  contained  in  a  wire  basket,  was  placed  in  a  septic 
tank  and  allowed  to  remain  there  for  one  month  and  twenty- 
seven  days,  from  October  4  to  December  31,  1900.  When 
taken  out  the  cloth  and  paper  were  still  intact,  but  so  rotten 
that  they  fell  to  pieces  when  touched. 

Cellulose  fermentation  is  an  anaerobic  process  that  in 
nature  is  going  on  at  all  times  in  manure  heaps  and  in 
vegetation  at  the  bottom  of  lakes  and  ditches,  where  it  is 
known  as  methane  or  marsh  gas  fermentation.  Cellulose 
fermentation  differs  from  alcoholic  fermentation  princi- 
pally in  the  products  of  the  two  ferments.  In-  cellulose 
fermentation,  methane  is  the  chief  product,  while  in  alco- 
holic fermentation  alcohol  is  the  chief  product. 

Before  cellulose  can  be  fermented,  the  organism  which 
effects  the  change  must  first  invert  the  cellulose,  that  is, 
break  it  down  by  means  of  its  enzymas  into  starch,  dextrin 
and  sugar.  It  is  then  changed  by  the  true  fermentation 
into  methane  and  carbon  dioxide,  fatty  acids  arising  as  a 
bi-product. 

Decomposition  of  Albumen — The  reduction  of  nitro- 
genous or  albuminous  materials  is  a  progressive  process 
that  is  both  aerobic  and  anaerobic,  and  results  in  the  lique- 
faction of  the  nitrogenous  solids  contained  in  sewage. 
This  process,  if  carried  too  far,  becomes  extremely  dis- 
agreeable, and  gives  rise  to  very  offensive  odors.  Anaero- 
bic decomposition  when  carried  to  the  disagreeable  stage, 
is  known  as  putrefaction,  familiar  examples  of  which  may 
be  cited  in  the  case  of  rotting  of  eggs  and  the  rank  putre- 
faction that  takes  place  in  water-tight  cesspools.  The 
products  of  nitrogenous  decomposition  are  ammonia  and 
carbon  dioxide,  which  may  remain  in  solution  or  in  gaseous 


22  SEWAGE    PURIFICATION    AND    DISPOSAL 

form  escape  to  the  atmosphere;  water,  which  passes  off  in 
the  effluent,  and  nitrous  acid,  the  only  organic  residue  left 
that  required  further  reduction.  Sewage  at  this  stage  of 
decomposition  contains  no  oxygen,  and  the  further  purifi- 
cation known  as  a  process  of  nitrification  is  effected  by 
aerobic  bacteria  and  must  be  carried  on  in  the  presence  of 
air -or  oxygen.  The  conditions  favorable  to  the  decompo- 
sition of  nitrogenous  matter  are,  a  temperature  above  50 
degrees  Fahr.  and  not  over  100  degrees  Fahr. ;  moisture; 
a  medium  that  is  not  either  strongly  acid  or  excessively 
alkaline,  but  is  preferably  slightly  alkaline,  and  conditions 
that  promote  the  exclusion  of  air.  To  bring  about  this 
last  condition  it  is  not  necessary  to  enclose  a  decomposition 
or  septic  tank,  as  experience  has  demonstrated  that  a  thick 
scum  capable  of  excluding  air  will  form  on  the  surface  of 
open  sewage  tanks. 

Nitrification — The  process  of  nitrification  is  effected  in 
two  stages :  first,  the  formation  of  nitrites  in  the  form  of 
nitric  acid  from  nitrous  acid,  and  second,  the  formation  of 
nitrates  from  the  combination  of  nitric  acid,  oxygen  and 
an  alkaline  base.  There  are  certain  conditions  necessary 
for  the  successful  nitrification  of  organic  matter.  The 
first  requisite  is  a  suitable  food.  Nitrifying  organisms 
cannot  thrive  in  the  presence  of  large  quantities  of  organic 
matter,  but  they  can  apparently  feed  upon  organic  matter, 
or  with  equal  ease  thrive  upon  inorganic  matter.  There 
must,  however,  in  either  case  be  present  a  certain  amount 
of  phosphate.  A  second  requisite  is  a  plentiful  supply  of 
oxygen,  without  which  nitrification  cannot  take  place.  In 
fact,  if  there  is  a  deficiency  of  oxygen  and  if  sufficient 
organic  matter  is  present  to  support  the  life  of  dentrifying 
organisms,  a  reverse  process  will  occur  which  instead  of 
building  up  breaks  down  with  an  evolution  of  nitrogen 
gas.  A  further  condition  for  nitrification  is  the  presence 
of  a  base  with  which  the  nitric  acid  can  combine  to  form 
nitrates.  Nitrification  can  take  place  only  in  a  feebly 
alkaline  medium ;  an  access  of  alkali  will  retard  the  proc- 
ess. The  final  requirement  of  nitrifying  organisms  is  a 


SEWAGE    PURIFICATION    AND    DISPOSAL  23 

favorable  temperature.  Nitrifying  bacteria  can  act  at  as 
low  a  temperature  as  37  degrees  Fahr.,  but  at  a  higher 
temperature  they  become  more  active.  The  temperature 
at  which  their  activity  becomes  sufficient  for  ordinary  nitri- 
fication is  54  degrees  Fahr. ;  their  activity  then  increases 
with  the 'temperature  until  99  degrees  Fahr.  is  reached, 
after  which  there  is  a  rapid  falling  off  of  the  activity  until 
at  122  degrees  Fahr.  it  almost  ceases,  and  at  131  degrees 
Fahr.  it  ceases  altogether.  Strong  light  as  well  as  high  or 
low  temperature  is  injurious  to  the  process.  Darkness  or 
diffused  light  is  good  for  nitrification.  The  seasons  of  the 
year  have  some  effect  on  the  process  of  nitrification, 
which  is  more  active  during  the  growing  months  of  May 
and  June  than  in  the  warmer  months  of  July  and  August. 
Denitrif ication — The  reverse  process  of  nitrification  will 
take  place  in  a  filter  bed  or  contact  bed,  if  sewage  high  in 
ammonia  compounds  but  devoid  of  oxygen  is  applied  to  a 
bed  that  is  not  well  aerated.  In  this  reverse  process, 
anaerobic  bacteria  take  the  oxygen  they  require  from  the 
nitrates,  thus  breaking  them  down  into  simple  compounds 
with  an  evolution  of  nitrogen  gas.  The  oxygen  liberated 
by  the  organisms  combines  with  the  carbon  to  form  car- 
bon dioxide,  a  portion  of  which  may  be  evolved  as  gas, 
while  the  remainder  combines  with  a  base  to  form  an  acid 
carbonate. 


ANALYSIS  OF  SEWAGE 


INTERPRETATION  OF  SEWAGE  ANALYSIS 


CHEMICAL    ANALYSIS 

Object  of  Sewage  Analysis — There  are  two  conditions 
which  are  determined  by  a  chemical  analysis  of  sewage :  One 
is  the  amount  of  organic  and  inorganic  matter  in  the  sew- 
age, and  the  other  is  the  stages  of  putrefaction  or  reduction 
the  organic  matter  has  undergone.  Both  conditions,  that 
is,  the  staleness  of  the  sewage  and  the  amount  of  organic 
and  inorganic  matter  contained,  are  determined  by  the  one 
set  of  chemical  examinations,  which  consist  of  reactions  to 
determine  the  amount  of  free  ammonia  and  albuminoid 
ammonia  both  in  suspension  and  in  solution,  the  amount 
of  chlorine,  the  total  solids,  nitrates,  and  the  oxygen 
consumed. 

In  a  chemical  determination  of  the  organic  matter  in 
sewage,  the  quantities  reported  are  only  an  index  of  the 
total  amount  and  cannot  be  considered  as  the  total  quantity 
present.  For  instance,  organic  matter  is  made  up  of  vari- 
ous compounds,  of  which  water  is  the  chief  constituent, 
usually  averaging  as  high  as  80  per  cent.  If  then  100,000 
grains  of  extremely  strong  sewage,  of  which  one-half  bulk 
is  organic  matter,  were  examined  chemically,  the  organic 
matter  would  be  determined  by  evaporating  to  dryness  a 
quantity  of  the  sewage  and  weighing  the  dry  residue;  as 
all  moisture  would  have  been  evaporated  from  the  organic 
matter,  the  residue  would  consist  of  but  20  per  cent,  of  the 
original  bulk,  and  this  20  per  cent,  is  all  that  would  be 
reported  by  the  chemists. 

Determination  of  Odor,  Color  and  Turbidity — In  sewage 
analysis,  the  odor,  color,  turbidity  and  other  physical 
characteristics  should  be  noted.  It  is  evident  that  an  efflu- 
ent which  emits  an  odor  cannot  be  well  purified.  Color 
should  not  be  noticeable  in  effluents  except  where  dye- 


SEWAGE    PURIFICATION    AND    DISPOSAL  25 

stuffs  from  industrial  wastes  are  treated  or  where  the 
water  supply  is  colored,  as  in  the  case  of  some  surface 
water  supplies  that  are  obtained  from  swampy  regions. 
Turbidity  is  an  indication  that  the  effluent  contains  large 
quantities  of  finely  disseminated  matter  in  suspension 
and  that  further  treatment  is  necessary  for  its  purifica- 
tion. 

Acidity  or  Alkalinity — Moderate  alkalinity  is  favorable 
for  bacterial  purification  of  sewage,  and  a  determination  of 
the  alkalinity  of  sewage  should  be  made  at  each  analysis, 
as  the  acidity  or  alkalinity  of  the  sewage  will  in  many 
cases  explain  the  cause  of  good  or  poor  effluent.  For 
instance,  if  nitrification  were  poor,  or  the  effluent  from  a 
septic  tank  cloudy,  and  at  the  same  time  the  sewage  were 
acid,  the  poor  liquefaction  in  the  septic  tank  and  poor  nitri- 
fication in  the  filter  beds  could  be  accounted  for  by  the 
acidity  of  the  sewage,  and  means  taken  to  neutralize  the 
acidity  by  the  addition  of  a  suitable  alkaline  base.  If,  on 
the  other  hand,  poor  reduction  and  nitrification  are 
obtained  with  a  moderately  alkaline  sewage,  the  cause  of 
the  poor  effluent  must  be  looked  for  elsewhere,  and  the 
cause  remedied. 

Temperature — The  temperature  of  both  the  sewage  and 
the  effluent  should  be  taken  at  the  time  of  collecting  sam- 
ples for  analysis.  Temperature  of  the  air  and  condition  of 
wind  or  calm  sunshine  or  cloudiness  are  also  valuable  data 
under  some  conditions.  As  certain  conditions  of  tempera- 
ture are  more  favorable  than  others  for  bacterial  putrefac- 
tion of  sewage,  it  is  obvious  that  poor  reduction  or  nitrifi- 
cation can  be  accounted  for  by  the  temperature,  which 
otherwise  could  not  be  explained. 

Determination  of  Solid  Matter — The  total  amount  of 
solid  matter,  both  organic  and  inorganic,  in  the  sewage  to 
or  effluent  from  purification  works  is  determined  by  the 
evaporating  to  dryness  of  a  certain  quantity  of  liquid,  and 
weighing  the  residue.  The  amounts  of  organic  and  inor- 
ganic matter  are  then  determined  by  igniting  the  dry 


26  SEWAGE    PURIFICATION    AND    DISPOSAL 

residue  and  subsequently  weighing  the  ash.  The  amount 
of  ash  gives  the  total  amount  of  inorganic  matter  in  a 
quantity  of  sewage  analyzed  and  the  difference  between 
the  dry  residue  and  the  ash,  which  difference  is  often 
reported  as  "loss  on  ignition,"  is  an  index  of  the  total 
amount  of  organic  matter  present. 

The  determination  of  total  solids  is  more  important 
than  might  at  first  appear,  for  it  indicates  not  only  the 
nitrogenous  materials  both  of  animal  and  of  vegetable  ori- 
gin, but  it  also  indicates  the  amount  of  carbon  or  non-nitro- 
genous matter  in  the  sewage,  while  all  other  determinations 
indicate  the  presence  of  only  one  constituent  of  sewage. 

The  amounts  of  solid  matter,  both  organic  and  inor- 
ganic, in  suspension  and  in  solution,  are  determined  by 
passing  through  filter  paper  a  certain  quantity  of  sewage. 
The  fluid  that  passes  through  the  filter  paper  is  then 
carefully  evaporated  to  dryness  and  the  proportions  of  the 
dry  residue  that  are  organic  and  inorganic  are  determined 
in  the  same  manner  as  in  the  case  of  total  dry  solids. 
These  indicate  respectively  the  quantity  of  matter  both 
organic  and  inorganic  that  is  in  solution  in  the  sewage. 

The  portion  of  sewage  held  back  by  the  filter  paper  is 
treated  in  a  similar  manner,  and  the  amounts  determined 
indicate  the  quantities  of  organic  and  inorganic  matter  in 
suspension  in  the  sewage. 

The  ratio  of  suspended  to  soluble  matter  is  an  index 
of  the  staleness  of  the  sewage,  for  the  fresher  the  sewage, 
the  more  matter  in  suspension  compared  with  that  in  solu- 
tion ;  and,  conversely,  the  more  matter  in  solution  compared 
with  that  in  suspension,  the  staler  must  be  the  sewage. 

Chlorine — Chlorine  is  present  in  sewage  in  the  form  of 
chlorides,  chiefly  of  sodium,  with  less  quantities  of  potas- 
sium and  ammonium.  Chlorine  is  a  constituent  of  common 
salt,  and  besides  being  used  extensively  in  the  household, 
it  enters  largely  into  the  composition  of  all  animal  tissue, 
and  forms  a  large  percentage  of  all  animal  excretions.  For 
instance,  as  may  be  seen  in  Table  I,  the  salts  in  urine  consti- 
tute more  than  one-third  of  the  entire  fluid. 


SEWAGE    PURIFICATION    AND    DISPOSAL  27 

TABLE  I — CONTENTS  OF  URINE 

Urea 512.4  grains 

Extractives  (pigment,  mucus,  uric  acid)        .  169.5  grains 
Salts  (chiefly  chlorides  of  sodium  and  potas- 
sium)            425.0  grains 

Total 1106.9  grains 

The  presence  of  chlorine  in  sewage,  above  the  normal 
amount  present  in  the  water  supply  of  that  locality,  indi- 
cates animal  pollution,  and  the  amount  of  chlorine  above 
the  normal  is  an  index  of  the  extent  of  pollution.  Ordinary 
water  supplies  contain  but  little  chlorine,  the  amount  gen- 
erally ranging  from  i  to  2  parts  per  100,000.  Weak  domes- 
tic sewage  contains  about  7  parts  per  100,000,  strong  sew- 
age contains  anywhere  from  7  parts  to  50  parts  per  100,000, 
while  sewage  of  average  strength  will  probably  show  about 
10  parts  of  chlorine  per  100,000  parts  of  water. 

The  amount  of  chlorine  in  sewage  is  not  affected  by 
purification,  and  the  determination  of  chlorine  in  an  efflu- 
ent should  correspond  approximately  with  the  amount  of 
chlorine  in  the  raw  sewage.  Any  discrepancy  in  the  two 
determinations,  not  due  to  error,  that  shows  less  chlorine 
in  the  effluent  than  is  present  in  the  sewage,  indicates  an 
infiltration  of  ground  water,  or  dilution  from  some  other 
source.  Frankland*  recommends  for  finding  the  quantity 
of  ground  water  diluting  sewage,  the  formula: 


In  which  x=the  volume  of  sub-soil  or  ground  water  diluting  a  vol- 
ume of  sewage 

a=chlorine  in  100,000  parts  of  sewage 
b= chlorine  in  100,000  parts  of  water 
c=chlorine  in  100,000  parts  of  effluent 

EXAMPLE — With  how  many  volumes  of  ground  water  has  sewage 
been  diluted  when  analysis  shows  20  parts  per  100,000  of  chlorine  in 
the  sewage,  10  parts  per  100,000  in  the  effluent  and  the  ground  water 
contains  2  parts  per  100,000  ? 

SOLUTION — Substituting  in  the  formula  the  values  given  in  the 
example : 

20—10     .  OK      .  . 

X=TT^ — jj-=1.25  volume  of  ground  water. 

*  Experimental  researches. 


28  SEWAGE    PURIFICATION    AND    DISPOSAL 

Determination  of  Organic  Nitrogen — The  organic  nitro- 
gen in  sewage  is  determined  by  testing  for  free  ammonia, 
albuminoid  ammonia,  nitrites  and  nitrates.  In  these 
determinate  only  the  total  amount  of  free  ammonia  is 
ascertained,  while  the  total  amount  of  albuminoid  ammo- 
nia, also  the  amounts  in  suspension  and  in  solution,  are 
separately  determined.  The  sum  of  the  amounts  of  albu- 
minoid matter  in  suspension  and  in  solution  should  equal 
the  total  amount  of  albuminoid  ammonia  in  the  sewage, 
while  the  sum  of  the  free  ammonia,  albuminoid  ammonia, 
nitrates  and  nitrites  indicates  the  amount  of  organic  nitro- 
gen present. 

Albuminoid  Ammonia — In  fresh  sewage,  most  of  the 
organic  nitrogen  is  present  as  albuminoid  ammonia,  and 
the  greater  part  of  it  is  in  suspension.  This  indicates  that 
but  little  time  has  elapsed  since  the  sewage  was  formed 
and  that  but  little  bacterial  action  has  taken  place.  In  the 
progressive  stages  of  sewage  purification,  the  next  step 
transforms  the  albuminoid  ammonia  from  suspension  into 
solution.  Albuminoid  ammonia  in  solution  when  in  greater 
quantity  than  in  suspension  then  indicates  bacterial  activity 
in  the  right  direction. 

Free  Ammonia — From  albuminoid  ammonia  in  solution 
the  nitrogenous  matter  in  sewage  becomes  and  is  deter- 
mined as  free  ammonia  both  in  suspension  and  in  solution. 
The  albuminoid  ammonia  in  sewage  decreases  until  the 
organic  nitrogen  is  practically  converted  into  free  ammo- 
nia. This  condition  marks  the  final  stage  of  anaerobic 
purification.  Ammonia  contains  no  oxygen,  while  nitrates 
contain  a  considerable  amount  of  oxygen;  therefore  to 
carry  the  process  of  purification  or  mineralization  further 
requires  a  well  aerated  environment;  and  an  effluent  in 
which  the  nitrogenous  matter  has  been  reduced  to  free 
ammonia  indicates  that  further  septic  or  anaerobic  action 
would  be  detrimental,  causing  it  to  putrefy  instead  of 
purify. 

In  Table  II  are  shown  average  analyses  of  effluents 
from  a  septic  tank  at  the  Experimental  Station  of  the  State 


SEWAGE    PURIFICATION    AND    DISPOSAL  29 

TABLE  II — FREE  AND  ALBUMINOID  AMMONIA  IN  SEWAGE  EFFLUENTS 


Parts  per  100,000 

0) 

Ammonia 

Year 

05 

Ed 

tt> 

Albuminoid 

Chlorine 

Oxygen 

Bacteria 
per  Cubic 

P, 

Centimeter 

S 

Total 

In 
Solution 

In  Sus- 
pension 

1898 

57 

4.86 

0.41 

0.32 

0.09 

10.11 

2.29 

324,500 

1899 

57 

4.03 

0.34 

0.25 

0.09 

7.00 

2.52 

577,100 

1900 

56 

4.61 

0.39 

0.25 

0.14 

9.93 

2.85 

1,209,500 

1901 

56 

4.90 

0.43 

0.29 

0.14 

10.40 

3.12 

671,000 

Board  of  Health  at  Lawrence,  Mass.  This  table  is  inter- 
esting chiefly  in  that  it  shows  the  almost  entire  reduction 
of  albuminoid  ammonia  in  suspension,  the  comparatively 
small  amount  in  solution  and  the  large  amounts  of  free 
ammonia  present.  The  data  in  the  table  indicate  a  good 
effluent  ready  for  aeration. 

Nitrites — The  formation  of  nitrites  marks  the  first 
stage  in  the  oxidation  of  nitrogenous  materials  that  have 
become  converted  into  ammonia  compounds  by  the  process 
of  decomposition.  Nitrites  are  found  only  in  traces  in  fresh 
sewage,  and  increase  in  quantity  as  the  sewage  becomes 
stale.  An  increase  in  the  quantity  of  nitrites  is  always 
accomplished  by  a  corresponding  decrease  in  the  nitroge- 
nous compounds,  free  and  albuminoid  ammonia.  Nitrites 
are  very  unstable  and  in  the  presence  of  air,  phosphates, 
moisture  and  an  alkaline  base  are  quickly  converted  into 
more  stable  nitrates. 

Nitrates — The  formation  of  nitrates  marks  the  final 
stage  in  the  purification  of  the  organic  nitrogen  contained 
in  sewage,  and  the  determination  of  nitrates  is  of  great 
importance  in  sewage  analysis.  Fresh  sewage,  as  a  rule, 
contains  no  nitrates  and  only  traces  of  nitrites,  while  puri- 
fied effluents  from  filter  beds  should  be  high  in  nitrates. 
As  the  process  of  oxidation  progresses,  that  is,  at  different 
depths  in  a  filter  bed,  the  nitrates  should  increase  and  the 
ammonia  compounds  correspondingly  decrease. 


30  SEWAGE    PURIFICATION    AND    DISPOSAL 

Oxygen  Consumed — The  determination  of  oxygen  con- 
sumed is  for  the  purpose  of  ascertaining  the  amount  of 
carbonaceous  or  non-nitrogenous  matter  in  sewage.  All 
of  the  determinations  before  mentioned  are  for  the  purpose 
of  finding  the  amount  of  organic  nitrogen  present,  but  gave 
no  indication  of  the  presence  of  hydro-carbons  or  other 
non-nitrogenous  compounds.  The  greater  the  amount  of 
carbonaceous  matter  in  sewage,  the  greater  will  be  the 
quantity  of  oxygen  consumed.  For  instance,  in  very  pure 
spring  or  lake  water  no  oxygen  will  be  consumed.  In 
Lake  Superior  water  from  .1  to  .2  parts  of  oxygen  to 
100,000  parts  of  water  might  be  consumed,  while  sewage 
that  contains  large  quantities  of  carbonaceous  matter  might 
absorb  as  high  as  150  parts  of  oxygen  to  100,000  parts  of 
water. 

BIOLOGICAL  EXAMINATIONS 

Bacteria  in  Sewage — The  degree  of  reduction  that 
sewage  has  undergone  can  approximately  be  determined 
from  the  number  of  bacteria  present.  In  fresh  sewage 
they  are  most  numerous,  in  some  cases  numbering  as  high 
as  forty-five  million  per  cubic  centimeter.  As  sewage 
passes  through  successive  stages  of  purification,  the  food 
supply  of  various  species  of  bacteria  becomes  exhausted 
and  they  cease  to  exist ;  thus  the  number  of  micro-organisms 
is  constantly  growing  less  until  in  the  effluent  from  septic 
tanks  they  will  average  about  600,000  per  cubic  centimeter, 
and  in  the  effluent  from  a  filter  bed  they  will  average 
about  100,000  per  cubic  centimeter,  although  under  favor- 
able conditions  they  might. become  reduced  to  2,000  per 
cubic  centimeter. 

Pathogenic  bacteria  in  sewage  are  exposed  to  more 
unfavorable  conditions  and  to  more  unsuitable  environ- 
ment than  the  saprophytic  bacteria,  and  it  is  presumed 
that  any  process  of  purification  which  reduces  the  number  of 
saprophytic  bacteria  will  reduce  in  an  even  greater  ratio 
the  parasitic  bacteria.  As  the  pathogenic  bacteria  in 
sewage  are  but  a  small  per  cent,  of  the  total  number 


SEWAGE    PURIFICATION    AND    DISPOSAL  31 

present,  the  number  of  saprophytic  bacteria  found  by  a 
biological  examination  of  sewage  is  a  fair  index  of  the 
number  of  pathogenic  variety  that  might  be  present. 

TESTS  FOR  EFFLUENTS 

Incubation  Test — Tests  are  now  generally  made  of  the 
effluent  from  purification  works.  The  one  commonly 
applied  is  known  as  the  incubation  test,  and  is  founded  on 
the  principle  that  a  good  water,  free  from  organic  matter, 
contains  oxygen  and  will  not  stagnate  or  become  foul  if 
kept  out  of  contact  with  air.  Pure  water  being  saturated 
with  air  will  not  absorb  oxygen,  but  impure  water, 
sewage  or  sewage  effluent  that  contains  organic  matter, 
if  corked  in  a  bottle  and  placed  in  an  incubator,  which 
for  five  days  is  kept  at  a  temperature  of  80  degrees 
Fahr.,  will  upon  being  exposed  to  the  permanganate 
test  absorb  an  amount  of  oxygen  proportionate  to  its 
impurities. 

An  effluent  capable  of  passing  the  incubator  test  is 
required  only  when  it  is  to  be  discharged  into  a  dry  ditch 
or  open  channel  leading  to  a  water  course.  When  an 
effluent  is  discharged  directly  into  a  stream  that  has  a  dry 
weather  flow  of  seven  times  the  volume  of  sewage,  the 
purification  of  the  effluent  need  not  be  carried  to  the  extent 
of  passing  the  incubation  test,  as  further  purification  will 
take  place  in  the  stream. 

Fish  Test — Fish  cannot  live  in  water  that  is  saturated 
with  chemicals,  contains  poison,  or  is  devoid  of  oxygen. 
An  illustration  of  this  may  be  cited  in  the  case  of  the 
Passaic  River  in  New  Jersey,  where  fish  have  been  killed 
or  driven  away  by  the  rank  pollution  of  the  river  by  sew- 
age. It  is  a  safe  provision,  therefore,  to  require  the  efflu- 
ent from  purification  works  to  be  of  such  a  character  that 
fish  can  healthfully  live  in  the  undiluted  effluent.  Then 
when  effluent  is  mixed  with  the  water  of  a  river  and 
diluted  with  several  times  its  volume  of  water,  it  will  be 
perfectly  harmless  to  fish  and  not  likely  to  cause  other 
nuisance. 


METHODS   OF   SEWAGE   PURIFICATION 


SEPTIC  PURIFICATION  OF  SEWAGE 


PRINCIPLES  OF   SEPTIC   PURIFICATION 
THE  SEPTIC  TANK 

Bacterial  and  Physical  Action  in  Septic  Tanks — In  the 

septic  process  of  purification,  sewage  is  passed  slowly 
through  a  vessel  or  reservoir,  called  a  septic  tank,  which 
contains  a  large  body  of  sewage  undergoing  the  various 
processes  of  fermentation  and  putrefaction.  The  tank  is 
built  in  such  manner  as  to  promote  the  growth  and  activity 
of  the  saprophytic  bacteria  on  which  the  process  depends 
for  the  reduction  of  the  organic  matter  in  the  sewage. 
When  discharged  into  a  septic  tank,  sewage  undergoes  a 
physical  separation  or  sedimentation,  in  which  the  heavier 
particles  are  drawn  to  the  bottom  by  the  force  of  gravity 
and  there  contribute  to  the  accumulated  sludge,  while  the 
lighter  particles  float  to  the  top  of  the  liquid,  thus  forming 
a  scum  on  the  surface  and  leaving  the  intermediate  depth 
comparatively  clear.  At  this  stage  of  purification  the  sew- 
age contains  a  greater  or  less  amount  of  air  or  free  oxy- 
gen, according  to  the  length  of  time  it  has  been  in  the 
sewers  and  subject  to  bacterial  action.  The  aerobic  bac- 
teria in  the  sewage,  however,  rapidly  deprives  the  liquid 
of  its  oxygen,  while  at  the  same  time  the  layer  of  scum 
on  the  surface,  and  the  aerobic  bacteria  therein  contained, 
prevent  oxygen  from  the  atmosphere  from  penetrating  to 
the  lower  depth  of  the  tank.  The  sewage  thus  being 
deprived  of  oxygen,  is  in  suitable  condition  for  anaerobic 
action,  which  is  the  most  effective  in  liquefying  solids,  and 
the  tank  then  becomes  the  seat  of  two  very  distinct 
actions.  In  the  interior  and  on  the  bottom  of  the  tank 
anaerobic  bacteria  attack  the  solid  matter,  both  nitrogenous 
and  carbonaceous,  and  convert  it  into  simple  compounds 
suitable  for  the  requirements  of  aerobic  bacteria.  On  the 


SEWAGE    PURIFICATION    AND    DISPOSAL  38 

surface  of  the  liquid,  in  the  presence  of  air,  on  the  other 
hand,  aerobic  bacteria  are  busy  reducing  to  still  simpler 
forms  the  products  liberated  by  the  anaerobic  bacteria, 
and  at  the  same  time  aerobically  reducing  the  scum  on  the 
surface  of  the  sewage. 

The  sludge  on  the  bottom  of  the  tank  is  subjected  to  a 
physical  as  well  as  to  a  biological  action.  Gases  produced 
by  the  liquefying  bacteria  in  the  sludge,  in  rising  to  the 
surface  of  the  liquid,  entangle  or  saturate,  as  the  case  may 
be,  some  of  the  solid  matter  on  the  bottom.  The  solids  so 
affected,  buoyed  up  by  the  gases,  rise  slowly  toward  the 
surface  of  the  sewage.  When  near  the  surface  the  gas 
becomes  liberated  and  the  solids  again  are  carried  by  the 
force  of  gravity  to  the  sludge  in  the  bottom  of  the  tank. 
This  physical  action  is  going  on  constantly,  night  and  day, 
in  a  septic  tank.  Bubbles  of  gas  carrying  sludge  can  at 
any  time  be  seen  rising  to  view  in  the  tank. 

During  the  vertical  movement  of  sludge  in  a  tank  it  is 
carried  in  a  horizontal  direction  toward  the  outlet,  a  dis- 
tance proportioned  to  the  rate  of  flow  through  the  tank, 
and  the  time  consumed  in  traversing  the  vertical  distance 
from  the  bottom  of  the  tank  to  the  surface  of  the  fluid  and 
back  again  to  the  bottom  of  the  tank.  For  example,  in  a 
tank  6  feet  deep  to  the  surface  of  the  liquid  and  96  feet 
long,  through  which  sewage  flows  at  the  rate  of  4  feet  an 
hour,  if  solid  matter  consumed  five  minutes  in  its  vertical 
movement  from  the  bottom  of  the  tank  to  the  surface  of 
the  liquid  and  back  again,  it  would  be  carried  during  each 
vertical  course  one  inch  toward  the  effluent  outlet,  and  as  the 
distance  traversed  in  each  oscillation  would  equal  6x2  =  12 
feet,  the  entire  distance  traversed  before  reaching  the  outlet 
would  be  12x1152+96=13,920  feet,  or  almost  2^/2  miles. 

The  septic-tank  process  of  sewage  purification  is  not  a 
complete  process  in  itself,  but  only  a  preparatory  step  that 
almost  completely  reduces  the  carbonaceous  matter,  but 
only  partially  purifies  the  nitrogenous  matter  contained 
in  the  sewage.  The  effluent  from  a  septic  tank  is  entirely 
devoid  of  oxygen,  and  when  freely  exposed  to  air  by  falling 


34  SEWAGE    PURIFICATION    AND    DISPOSAL 

in  a  cascade  over  baffle  plates,  or  sprayed  into  the  air,  it 
almost  instantly  will  absorb  70  to  75  per  cent,  of  the  air  it 
can  absorb.  Subsequent  treatment  of  septic  effluent 
therefore  should  consist,  first,  of  a  thorough  aeration  and 
then  an  intermittent  application  in  thin  films  to  porous, 
well-aerated  soil,  sprayed  continuously  over  filter  beds  of 
coarse  material,  or  subjected  to  aerobic  treatment  in  con- 
tact beds,  so  nitrifying  bacteria  can  reduce  the  ammonia 
compounds  to  stable  nitrates. 

The  odors  in  the  neighborhood  of  "septic  tanks  are 
usually  noticeable  at  some  little  distance  from  the  tanks  in 
the  direction  in  which  the  wind  is  blowing.  Ordinarily, 
however,  the  odors  are  not  seriously  objectionable  even  on 
the  site  of  the  purification  works.  Bad  odors  generally 
result  from  the  treatment  of  sewage  containing  relatively 
high  quantities  of  sulphur  compounds,  such  as  the  acid 
waste  from  factories.  They  may  likewise  arise  from  the 
treatment  of  sewage  from  communities  in  which  the  water 
is  hard,  due  to  sulphates  of  lime  or  magnesia. 

Covered  and  Uncovered  Septic  Tanks— So  far  as  the 
effectiveness  of  septic  tanks  is  concerned,  there  is  no  dif- 
ference between  the  results  obtained  in  mild  climates  by 
the  use  of  closed  tanks  and  those  obtained  by  the  use 
of  open  tanks.  In  open  tanks,  as  in  closed  tanks,  the  scum 
that  forms  on  the  surface  of  the  liquid,  together  with  the 
aerobic  bacteria  existing  within  the  scum,  prevents  oxygen 
penetrating  to  the  lower  depth  of  the  tanks.  The  scum, 
furthermore,  forms  a  screen  to  shut  out  light  which  would 
interfere  with  the  bacterial  processes  within ;  the  turbidity 
of  the  liquid  also  prevents  light  rays  from  penetrating 
to  any  great  depth,  even  though  they  should  pierce  the 
upper  crust  or  find  entrance  where  the  crust  is  broken  or 
removed.  It  is  probable  that  on  the  liquid  in  open  septic 
tanks  a  thicker  crust  will  form  than  is  likely  in  a  closed 
tank,  where  darkness  encourages  aerobic  action  on  the  sur- 
face scum  that  no  doubt  is  interfered  with  in  open  tanks 
by  the  direct  rays  of  the  sun.  A  thick  scum  on  the  liquid 
in  open  septic  tanks  no  doubt  is  beneficial,  not  only  in 


SEWAGE    PURIFICATION    AND    DISPOSAL  35 

excluding  air  and  light,  but  also  in  maintaining  an  even 
temperature,  throughout  the  year,  of  the  sewage  contained 
in  the  tank.  As  a  rule,  a  thicker  scum  will  form  during 
the  summer  months  than  during  winter  weather. 

A  comparison  of  the  results  obtained  by  treating  in 
open  and  in  closed  septic  tanks  sewage  taken  at  the  same 
time  from  a  common  source,  can  be  found  in  Table  III, 
which  shows  the  average  results  obtained  by  Mr.  Fowler, 
of  Manchester,  England,  in  a  series  of  analyses  made  daily 
for  a  period  of  one  month.  The  results,  as  may  be  seen, 
are  almost  identical. 

While  there  is  no  difference  in  efficiency  between  open 
and  closed  septic  tanks  operated  in  mild  climates,  in  cold 

TABLE  III — COMPARISON  OF  RESULTS  FROM  OPEN  AND  CLOSED 
SEPTIC  TANKS 


Parts  in  100,000 

Open 
Tank 

Closed 
Tank 

Free  ammonia  
Albuminoid  ammonia 

3.20 
0  50 

3.10 
0  51 

Oxygen  consumed 

8  46 

8  43 

Chlorine    ....          ....          .          ... 

16.40 

16  10 

climates  the  closed  tank  will  be  found  to  possess  distinct 
advantages  over  open  tanks.  The  advantages  consist  in 
the  exclusion  of  snow,  which  in  northern  latitudes  is  no 
inconsiderable  amount,  and  in  the  maintaining  of  a  higher 
temperature  more  suitable  for  the  activity  of  bacteria  than 
can  be  maintained  in  an  open  tank.  In  mild  climates, 
when  the  additional  expense  of  covering  septic  tanks  need 
not  be  considered,  a  closed  tank  will  prove  the  more  satis- 
factory, as  it  conceals  from  sight  the  fermenting  and 
putrefying  mass  of  sewage,  prevents  the  nuisance  of  odors, 
protects  the  sewage  from  flies,  and  is  a  protection  against 
wind  and  rain.  In  warm  climates  a  wooden  covering  freely 
ventilated  will  prove  quite  satisfactory.  Covered  tanks 
should  be  provided  liberally  with  ventilation  ducts  or  out- 
lets for  the  escape  of  gas  liberated  by  the  decomposition 


36 


SEWAGE    PURIFICATION    AND    DISPOSAL 


of  sewage.  If  provision  is  not  made  for  the  removal  of 
gas  as  formed,  when  a  sufficient  quantity  has  accumulated, 
if  accidentally  ignited  it  is  liable  to  cause  a  disastrous 
explosion. 


CONSTRUCTION  OF  SEPTIC  TANKS 

Description  of  a  Septic  Tank  —  A  small  covered  septic 
tank  is  shown,  diagrammatically,  in  Fig.   i.   The  tank  is 


. 


^^".-^(^^^>y.-i^^^-^^/-^^.'<.*.A  -  -\ 
!••-.«- :••/•'«.•'.:•••.'«»•'.  «•••  m  _  ^.-«- ;  »-:  ••  u 


Fig.  1 


separated  by  a  wall,  a,  into  two  compartments,  the  septic 
tank  proper,  b,  and  a  collecting  and  discharge  chamber,  c, 
generally  called  a  dosing  chamber.  Sewage  enters  the 
septic  tank  through  the  inlet,  d,  which  is  turned  down  and 
submerged  so  sewage  will  not  disturb  the  surface  scum,  e. 
From  the  septic  tank,  the  effluent  overflows  the  weir,  or 
wall  into  the  dosing  chamber.  One  of  the  baffle  boards, 
/,  deflects  the  flow  of  sewage  towards  the  bottom  of  the 
tank,  while  the  other,  which  extends  down  about  3  feet 
below  the  surface  of  the  liquid,  prevents  the  surface  scum 
from  being  washed  over  with  the  effluent  and  insures  the 


SEWAGE    PURIFICATION    AND    DISPOSAL  37 

discharge  from  the  tank  being  taken  from  near  the  center 
level  where  the  sewage  is  most  clear.  When  baffle  boards 
are  used  they  should  be  spaced  about  10  feet  apart.  The 
board  nearest  the  inlet  should  project  a  few  inches  above 
the  line  of  flow  and  to  within  2^  feet  of  the  bottom  of  the 
tank.  The  middle  baffle  board  should  be  set  with  its  upper 
edge  about  18  inches  below  the  surface  and  its  lower  edge 
about  1 8  inches  above  the  bottom  of  the  tank.  The  scum 
board  near  the  outlet  should  extend  a  few  inches  above 
high  water  level,  and  the  bottom  edge  should  be  midway 
between  the  surface  of  sewage  and  the  bottom  of  the  tank. 
A  valved  sludge  pipe,  gy  provides  means  for  draining  off 
sludge  from  the  tank  without  throwing  the  tank  out  of 
service.  The  floor  of  the  tank  is  made  sloping  towards  this 
outlet  to  facilitate  the  washing  out  of  the  sludge.  A  sludge 
pipe  from  a  septic  tank  is  desirable  only  when  the  tank  is 
situated  at  such  a  level  that  the  sludge  can  be  flushed  by 
gravity  to  sludge  beds  located  at  a  lower  elevation,  or,  in 
case  the  sludge  cannot  be  flushed  by  gravity,  when  the  puri- 
fication plant  is  of  such  a  size  as  to  warrant  the  installation 
of  sludge  pumps. 

The  dosing  tank,  c,  can  be  omitted  from  a  septic  tank 
when  the  effluent  is  discharged  into  a  stream;  when,  how- 
ever, the  effluent  is  subsequently  treated  by  filtration,  as 
effluents  invariably  should  be,  the  dosing  chamber  should 
be  so  proportioned  to  the  filter  beds  that  one  dose  will 
properly  flood  the  filter  area.  The  floor  of  the  dosing  cham- 
ber is  made  sloping  towards  the  center  where  is  located 
the  valved  outlet,  h,  which  should  be  cross-connected 
to  the  discharge  to  the  filter  beds,  and  to  a  sewer  out- 
fall, so  that  the  effluent  can  be  discharged  direct  at  the 
place  of  disposal,  or  supplied  continuously  to  the  filter 
beds  during  repairs  to  the  automatic  siphon.  The  wall 
separating  the  septic  compartment  and  dosing  chamber 
should  be  made  of  sufficient  strength  to  hold  back  the 
dammed-up  liquid  in  the  septic  tank,  and  withstand  the 
varying  strains  caused  by  slowly  filling  and  quickly  empty- 
ing the  dosing  compartment.  The  siphon  apparatus 


38  SEWAGE    PURIFICATION    AND    DISPOSAL 

shown  in  the  illustration  is  a  Miller  Automatic  Siphon, 
and  is  operated  in  the  following  manner:  When  sewage 
overflows  the  wall,  a,  into  the  dosing  chamber  it  rises 
in  the  bell  of  the  siphon  and  overflows  into  the  trap, 
which  it  seals,  thus  confining  the  air  in  the  space,  z,  which 
forms  the  long  leg  of  the  siphon.  As  .sewage  effluent  then 
rises  in  the  dosing  chamber  it  compresses  the  confined 
air  in  the  long  leg,  thus  forcing  the  water  down  on  one 
side  and  up  on  the  other,  as  shown  in  the  illustration, 
until  the  compressed  air  in  i  is  just  about  to  escape  under 
the  bend  that  forms  the  dip  of  the  trap.  Any  further 
flow  of  liquid  into  the  dosing  chamber  will  then  compress 
the  water  in  i  so  that  the  confined  air  can  escape  from  the 
trap  carrying  with  it  some  of  the  water;  as  the  air  escapes 
from  i  the  space  fills  with  water  from  the  dosing  chamber, 
thus  filling  the  long  leg  of  the  siphon  which  immedi- 
ately is  thrown  into  operation  and  aspirates  the  contents 
from  the  dosing  chamber.  When  the  effluent  in  the 
dosing  chamber  is  lowered  to  the  level  of  the  mouth  of 
the  air  pipe,  /,  the  siphonage  is  slowly  broken  by  the 
admission  of  air  through  the  pipe.  In  addition  to  serving 
as  a  vent  to  break  the  siphon,  the  air  pipe,  /,  permits  the 
escape  of  air  from  the  space,  z,  when  sewage  is  flowing 
in  to  fill  the  dip  of  the  trap,  a  condition  that  is  necessary 
for  the  successful  operation  of  the  siphon. 

When  effluent  in  the  dosing  chamber  reaches  the  level 
of  the  mouth  of  this  air  pipe  the  mouth  of  the  pipe  becomes 
sealed,  thus  confining  the  air  in  the  long  leg  of  the  siphon. 
The  confined  air  in  the  long  leg  of  the  siphon  then 
becomes  compressed,  and  depresses  the  water  in  the  trap  in 
direct  proportion  to  the  rise  of  sewage  in  the  dosing 
chamber. 

The  depth  of  liquid  in  the  dosing  chamber  that  will 
cause  the  automatic  siphon  to  discharge,  depends  on  and 
can  be  gauged  by  the  depth  of  water  in  the  short  leg,  k, 
of  the  trap. .  It  is  obvious  that  the  column  of  water  inside 
of  the  bell  will  balance  that  on  the  outside  of  the  bell,  and 
that  the  column  of  water,  £,  in  the  short  leg  of  the  trap  will 


SEWAGE    PURIFICATION    AND    DISPOSAL  39 

balance  an  equal  column  of  water  in  the  dosing  chamber 
above  the"  level  of  water  in  the  bell.  Any  increase  in  the 
depth  of  water  in  the  dosing  chamber  destroys  the  equilib- 
rium of  the  two  columns  and  brings  the  siphon  into  full 
action.  It  follows  therefore  that  the  short  leg  of  the  trap 
must  be  of  just  such  length  as  there  will  be  depth  of  liquid 
above  the  bell  in  the  dosing  chamber  when  the  siphon  is 
brought  into  operation. 

The  short  leg  of  the  trap  projects  abruptly  into  the 
discharge  pipe  to  permit  the  instantaneous  escape  of  water 
from  the  outlet.  This  is  a  necessary  provision  without 
which  water  would  not  escape  with  sufficient  velocity  to 
disturb  the  equilibrium  of  the  two  columns  and  set  the 
siphon  in  operation. 

An  overflow  pipe  is  provided  to  carry  off  the  effluent  in 
case  the  siphon  becomes  obstructed.  This  overflow  pipe 
serves  also  as  a  vent  pipe  through  which  air  can  circulate 
from  the  outlets  at  the  filter  beds  or  other  place  of  disposal, 
up  to  and  through  the  perforated  covers  of  the  manholes. 

Capacity  of  Septic  Tanks — Ordinarily,  septic  tanks 
should  have  a  capacity  equal  to  the  dry  weather  flow  of 
sewage  for  twenty-four  hours.  The  dry  weather  flow  is 
assumed  as  a  basis,  because  during  rainy  weather  or  imme- 
diately after  a  storm,  rain  water  entering  through  rain- 
water openings  to  the  street  and  from  rain  leaders  on 
buildings,  also  ground  water  infiltering  to  the  sewers 
from  the  soil,  add  considerably  to  the  bulk  of  sewage  to 
be  handled  at  a  purification  works.  If  plants  were  made 
large  enough  to  care  for  storm  water  in  addition  to  the 
usual  domestic  sewage,  they  would  be  too  large  for  ordi- 
nary purposes;  so  to  obtain  the  best  results,  provision  is 
made  to  purify  the  ordinary  dry-weather  flow,  and  addi- 
tional provision  made  to  care  for  the  storm  water  without 
subjecting  it  to  the  usual  purification  process.  In  pro- 
viding for  the  dry-weather  flow,  it  should  be  borne  in 
mind  that  tanks  sometimes  sludge  to  such  an  extent  that 
only  about  75  per  cent,  of  their  capacity  is  available  for 
storage,  and  this  limitation  in  size  should  be  taken  into 


40  SEWAGE    PURIFICATION    AND    DISPOSAL 

consideration  when  proportioning  a  tank.  The  character 
of  the  sewage  will  have  a  great  deal  to  do  with  the  amount 
of  sludge  formed,  and  where  a  sewage  is  strong  and  con- 
tains an  unusual  quantity  of  cellulose  material,  an  addi- 
tional capacity  of  about  25  per  cent,  would  probably  be 
advisable,  while  with  a  weak  domestic  sewage  of  ordinary 
composition,  a  tank  with  a  gross  capacity  of  twenty-four 
hours'  dry  weather  flow  will  no  doubt  be  found  sufficient. 
Many  tanks  have  been  installed  with  capacities  of  only 
twelve  to  eighteen  hours'  flow,  but  with  few  exceptions 
they  have  been  found  too  small,  and  under  the  most  favor- 
able conditions  are  producing  effluent  much  inferior  to 
those  with  twenty-four  hours'  capacity.  Too  long  storage, 
on  the  other  hand,  must  be  avoided.  In  some  cases  where 
the  effluent  is  to  be  subsequently  treated  by  sprinkling 
filters  followed  by  slow  sand  filtration,  a  period  of  sedi- 
mentation or  septic  action  of  ten  or  twelve  hours  might 
prove  sufficient,  and  permit  the  treatment  of  a  greater 
quantity  of  sewage  with  the  production  of  a  satisfactory 
effluent.  ^ 

Velocity  of  Flow  through  Septic  Tanks — In  the  septic 
process  of  purification,  in  order  to  secure  the  best  results, 
it  is  necessary  that  the  sewage  flow  continuously  into  the 
tank,  to  provide  a  constant  supply  of  food  for  the  reducing 
micro-organisms,  while  at  the  same  time  the  effluent  from 
the  tank  is  continuously  withdrawn  to  carry  off  the  toxic 
enzymes  elaborated  by  the  bacteria.  The  flow  of  sewage 
through  the  tank,  while  continuous  must  not  be  at  too 
great  velocity,  or  sufficient  time  will  not  elapse  during 
the  passage  for  sedimentation  and  bacterial  action  to  prop- 
erly perform  their  functions.  It  has  been  found  in  prac- 
tice, both  in  the  treatment  of  turbid  waters  and  in  the 
purification  of  sewage,  that  twenty-four  hours  is  a  suffi- 
cient length  of  time  for  sedimentation.  Any  particles  that 
will  settle  will  do  so  in  that  time,  while  a  greater  period 
of  time  adds  considerably  to  the  size  of  tank  required  with- 
out affecting  a  corresponding  clarification.  Furthermore, 
a  longer  period  of  storage  in  a  septic  tank  is  liable  to  carry 


SEWAGE    PURIFICATION    AND    DISPOSAL  41 

the  putrefactive  process  so  far  that  the  products  will  check 
the  reducing"  organisms  and  produce  an  effluent  difficult 
to  nitrify.  This  is  more  liable  to  occur  when  treating  a 
strong  than  when  treating  a  weak  sewage.  In  practice, 
the  length  of  time  that  sewage  remains  in  a  septic  tank 
varies  from  eighteen  to  twenty-four  hours.  In  some  im- 
properly proportioned  purification  works,  a  much  less 
period  of  sedimentation  is  provided  for  with  corre- 
spondingly inferior  results.  The  length  of  time  required 
for  sedimentation  and  bacterial  action  in  a  septic  tank 
varies  somewhat  with  the  staleness  of  the  sewage.  Ordi- 
narily, for  fresh  sewage,  it  may  be  stated  as  a  rule  that 
twenty-four  hours'  time  should  be  allowed;  on  the  other 
hand,  when  the  sewage  has  flowed  through  sewers  for 
three  or  four  hours,  and  arrives  at  the  purification  works 
in  a  stale  condition,  a  corresponding  length  of  time  can  be 
deducted  from  its  length  of  stay  in  or  passage  through  the 
tank.  In  designing  a  purification  plant,  however,  it  is 
good  provision  to  allow  for  a  flow  of  twenty-four  hours 
through  the  tank,  then,  upon  the  growth  of  the  commu- 
nity, when  sewage  must  be  passed  through  at  a  higher 
velocity,  it  can  be  so  done  without  materially  decreasing 
the  effectiveness  of  the  purification. 

The  actual  velocity  of  flow  through  a  tank  should  not 
be  greater  than  one  inch  per  minute,  and  should  be  at 
least  one-half  inch  per  minute. 

Uniformity  of  Flow  in  Septic  Tanks — Not  less  impor- 
tant than  the  velocity  of  flow  is  the  uniformity  of  flow 
through  septic  tanks.  If  arrangements  are  not  made  to 
insure  uniformity  of  flow  throughout  the  whole  cross  sec- 
tion of  the  tank,  the  sewage  is  likely  to  flow  in  a  narrow 
stream  or  channel  with  greater  velocity  than  if  the  flow 
were  uniform.  In  order  to  insure  uniformity  of  flow,  the 
inlet  to  a  septic  tank  should  be  so  designed  that  the  sewage 
will  be  spread  over  the  entire  cross  section.  This  is 
best  done  by  providing  several  branch  inlets  to  the  tank, 
opening  at  the  same  level  but  at  various  distances  from 
the  center  line.  Such  an  inlet  not  only  distributes  the 


SEWAGE    PURIFICATION    AND    DISPOSAL 


sewage  uniformly,  but  reduces  its  velocity  on  entering 
so  as  not  to  produce  eddies  or  currents  to  disturb  the 
sludge.  The  effluent  from  a  septic  tank  usually  flows 
over  a  weir  which  extends  clear  across  the  tank,  this 
insuring  a  uniform  flow  from  the  entire  cross  section. 

In  Table  IV  will  be  found  the  average  purification 
affected  by  different  lengths  of  flow  through  open  septic 
tanks. 

TABLE  IV — AVKRA<;K  OF  ANALYSES  ILLUSTRATING  THE  EFFECT  UK  DIF- 
FERENT RATES  OF  FLOW  THRUUGH  OPEN  SEPTIC  TANKS  (LEEDS,  IDO.Y) 


12  Hours' 

24  Hours' 

48  Hours' 

72  Hours' 

Flow 

Flow 

Flow 

Flow 

Parts 
per 
mil- 
lion 

Puri- 
fica- 
tion 
(per 
cent.) 

Parts 
per 
mil- 
lion 

Puri- 
fica- 
tion 
(per 

cent.) 

Paris 
per 
mil- 
lion 

Puri- 
fica- 
tion 
(per 
cent.) 

Parts 
per 
mil- 
lion 

Puri- 
fica- 
tion 
(per 
cent.) 

Total  solids     

1,250 

1,110 

1,120 

1,050 

Suspended  solids     .     .     . 
Nitrogen  as- 

272 

52 

102 

71 

155 

73 

141 

76 

Free  ammonia    .     .     . 

18.2 

88 

17.5 

24 

18.8 

19 

90,8 

37 

Albuminoid  ammonia 

6.3 

50 

5.2 

58 

4.5 

D  I 

4 

52 

Oxygen    consumed   in  4 

hours  at  80°  F  .     .     .     . 

T4.2 

45 

68.8 

49 

01.2 

55 

51.1 

66 

Shape  and  Dimensions  of  Septic  Tank — There  is  no  cer- 
tain shape  which,  better  than  another,  conduces  to  the 
efficiency  of  a  septic  tank.  For  economic  reasons,  how- 
ever, tanks  are  built  square  in  preference  to  rectangular 
and  are  built  round  in  preference  to  oval,  as  these  shapes 
enclose  greater  areas  for  equal  lengths  of  wall.  To  con- 
form to  the  topography  of  the  land  or  to  the  shape  of  a 
purification  field,  septic  tanks  are  sometimes  made  irreg- 
ular in  shape  or  in  depth.  This  irregularity,  however, 
while  it  adds  to  the  cost  of  the  plant,  in  no  way  affects 
its  efficiency. 

Within  certain  limits  the  depth  of  a  septic  tank  can  be 
adapted  to  suit  the  requirements  of  each  installation,  pro- 
vided it  is  made  not  over  10  or  12  feet  deep.  If  a  tank 
is  made  of  greater  depth,  difficulty  will  be  experienced 
in  securing  a  uniform  movement  of  the  sewage,  unless 


SEWAGE    PURIFICATION    AND    DISPOSAL  43 

submerged  baffle  plates  are  provided  to  deflect  the  fluid.  Fur- 
thermore, a  greater  depth  to  a  tank  will  increase  the  difficulty 
of  removing,  and  the  power  required  to  remove,  the  sludge, 
either  by  hand  or  by  power,  or  the  tanks  would  have  to  be 
located  at  a  higher  elevation  to  secure  gravity  discharge  of 
sludge  to  the  sludge  beds.  As  the  sewerage  system  grades 
from  the  entire  drainage  area  towards  the  purification  plant, 
which  usually  is  situated  at  a  very  low  level,  near  the  place 
of  disposal,  the  depth  that  a  tank  can  be  extended  gener- 
ally is  limited  by  the  available  fall  from  the  bottom  of  the 
tank  to  the  sewer  outfall,  or  to  the  filter  beds. 

The  construction  of  tanks  that  are  too  shallow  should 
also  be  avoided,  as  under  such  conditions  the  flow  of  sewage 
is  liable  to  stir  up  the  sludge  and  carry  off  part  of  it  in  the 
effluent.  Five  feet  is  probably  the  least  depth  that  will 
produce  satisfactory  results. 

Reduction  of  Sludge  in  Septic  Tanks — There  is  no  way 
of  predetermining  the  amount  or  percentage  of  sewage 
that  will  be  reduced  or  destroyed  in  a  septic  tank.  It  is 
fair  to  assume,  however,  that  under  the  most  favorable 
conditions  a  tank  rightly  proportioned  with  sufficient  depth, 
and  having  a  capacity  of  24  hours'  storage,  through  which 
the  sewage  flows  with  a  velocity  not  greater  than  one  inch 
per  minute,  if  maintained  at  a  uniformly  high  temperature, 
will  reduce  at  least  90  per  cent,  of  the  sludge  in  domestic 
sewage.  Under  ordinary  conditions  a  septic  tank  will  re- 
duce from  70  per  cent,  to  90  per  cent,  of  the  sludge  in  an 
average  domestic  sewage,  free  from  manufacturing  waste. 
Where  conditions  are  more  favorable  and  the  septic  tank 
is  properly  designed,  the  sludge  will  entirely  disappear. 
Such  has  been  the  case  at  Lawrence,  Mass.,  where  the 
sludge  is  entirely  reduced  and  where  for  over  four  years 
no  sludge  has  been  taken  from  the  tanks.  It  should  be 
remembered,  however,  that  for  the  first  three  or  four 
months  that  a  tank  is  in  service  there  will  be  an  accumu- 
lation of  sludge  during  the  period  of  seeding  and  ripening. 
This  period  can  be  reduced  somewhat  by  seeding  a  new  tank 
with  surface  scum  and  sludge  from  a  ripe  tank. 


44  SEWAGE    PURIFICATION    AND    DISPOSAL 

In  the  average  septic  tank  treating  domestic  sewage,  it 
can  be  assumed  that  of  the  matter  suspended  in  the  sewage, 
about  one-third  passes  through  the  tank  and  appears  in  the 
effluent;  about  one-third  is  liquefied  or  gasified,  and  the 
remaining  one-third  remains  as  sludge.  The  sludge,  how- 
ever, in  most  cases  can  be  reduced  to  about  one-tenth  by 
proper  design;  and  in  localities  where  the  temperature  of 
the  sewage  is  78  degrees  to  90  degrees  Fahr.  there  will  be  a 
complete  reduction  of  the  organic  matter. 


SEPTIC  TANK  DETAILS 

Detritus  Tank — In  most  systems  of  sewage  purification, 
whether  septic  tanks,  intermittent  filters,  contact  beds  or 
chemical  precipitation  plants,  a  tank  known  as  a  detritus 
tank  is  provided,  in  which  to  catch  heavy  mineral  or 
metallic  substances  that  might  have  been  washed  into  the 
sewers  and  carried  along  by  the  force  of  the  flowing  liquid 
to  the  purification  works.  Detritus  tanks  are  not  intended 
to  store  any  considerable  quantity  of  sewage,  but  are  more 
in  the  nature  of  traps  with  a  depression  below  the  level  of 
the  bottom  of  the  sewer.  When  anything  of  greater  spe- 
cific gravity  than  water  reaches  the  detritus  tank,  it 
immediately  sinks  to  the  bottom  where  it  remains  until  re- 
moved by  mechanical  means.  A  detritus  tank  should  be  of 
sufficient  length  so  that  particles  of  but  slightly  greater 
specific  gravity  than  water  cannot  be  carried  across  the 
tank  by  the  momentum  of  the  sewage.  The  outlet  from 
a  detritus  tank  to  the  purification  works  generally  is 
provided  with  a  screen  to  hold  back  any  large  floating 
objects  that  are  difficult  to  liquefy  and  that  if  not  re- 
moved might  interfere  with  the  proper  working  of  the 
.  plant. 

Dosing  Tanks — The  size  of  a  dosing  tank  depends  to  a 
great  extent  upon  where  the  effluent  is  to  be  discharged. 
If  the  effluent  is  to  be  discharged  into  a  stream  or  lake,  the 
siphon  and  dosing  chamber  may  be  dispensed  with  and  the 


SEWAGE    PURIFICATION    AND    DISPOSAL 


45 


effluent  allowed  to  flow  continuously  into  the  water.  If, 
however,  a  dosing  chamber  be  used,  the  siphon  should  be 
set  to-  discharge  at  frequent  intervals,  as  small  doses  at 
frequent  intervals  would  conduce  to  better  dilution  of  the 
effluent.  When,  on  the  other 
hand,  the  effluent  is  to  be 
treated  subsequently  by  fil- 
tration on  intermittent  filter 
beds,  the  dosing  tank  should 
have  a  capacity  in  proportion 
to  the  size  of  the  filters.  For 
instance,  if  the  dosing  cham- 
bers have  a  capacity  of  six 
hours'  storage,  the  filter  beds 
should  be  proportioned  to 
hold  a  six-hour  discharge. 

In  calculating  the  period 
of  storage  in  septic  tanks,  the 
time  required  to  fill  the  dos- 
ing chamber  may  be  de- 
ducted from  the  total  period. 
The  dosing  chamber  shown 
in  connection  with  the  septic 
tank,  Fig.  i,  is  extended  the 
full  depth  of  the  tank.  This 
depth,  however,  is  unnec- 
essary, and,  when  there  is  but  little  fall  from  the  septic 
tank  to  the  filter  beds,  the  dosing  chamber  can  be  made 
shallower  and  make  up  in  area  the  capacity  required.  When 
necessary,  the  bottom  of  the  dosing  chamber  can  be  within 
12  or  1 8  inches  of  the  level  of  the  outlet  to  the  tank. 


Fig.  2 


SEWAGE  SCREENS 

Removable  Screens — Sewage  screens  for  small  purifica- 
tion plants  are  generally  some  modification  of  the  screens 
shown  in  Figs.  2,  3  and  4.  It  is  a  point  of  economy 
when  specifying  screens  to  stipulate  that  they  be  of  some 


46 


SEWAGE    PURIFICATION   AND    DISPOSAL 


non-corrodible  material  that  will  not  quickly  be  eaten  away 
by  the  chemical  action  of  the  sewage.  Some  purification 
plants  have  a  lattice  work  made  of  iron  bars  bedded  in  the 


masonry.  With  such  an  arrangement,  the  screen  must  be 
kept  free  from  accumulations  by  removing  the  clogging 
material  by  hand  labor.  This  is  usually  done  by  scraping  the 
accumulations  away  with  hand  rakes,  a  process  which  gener- 
ally entails  considerable  labor.  With  removable  screens  on 
the  other  hand,  when  one  screen  becomes  clogged  with 
deposits  that  screen  can  be  removed,  another  screen  substi- 
tuted, and  the  old  one  laid  out  to  dry,  as  drying  facilitates 
the  removal  of  the  deposits.  A  removable  screen  is  shown 
in  Fig.  2.  Usually  two  or  more  sets  of  grooves  are  provided, 
so  that  a  fresh  screen  can  be  lowered  into  place  before  the 
old  one  is  removed. 

Mechanically  Operated  Screens — Mechanically  operated 
screens  have   been  successfully  operated   at  large   plants 


SEWA 


IFICATION    AND    DISPOSAL 


where  power  is  required  for  other  purposes.  The  expense 
of  operating  mechanical  screens  would  hardly  be  war- 
ranted however  in  plants  of  small  capacity  or  where  sewage 
with  but  little  coarse  matter  is  treated.  A  mechanical 
screen  that  is  in  use  in  Glasgow,  Scotland,  is  shown  in 
Fig.  3.  This  screen  is  inclined  at  an  angle  of  45  degrees, 
and  is  run  at  a  speed  of  14  revolutions  per  minute.  The 
face  of  the  screen  is  provided  with  buckets  to  lift  out 
floating  objects  which  are  carried  over  the  top  axis  and 
deposited  in  a  trough,  from  which  they  are  pushed  by 
hand  into  a  bucket  located  in  a  sump.  The  screen  is 
operated  by  chains  and  sprocket  wheels. 


'  G  '. 


Fig.  4 

Detritus  Tank  and  Screen  Chamber — An  illustration 
of  a  detritus  tank  and  screen  chamber  is  shown  in  per- 
spective in  Fig.  4.  Sewage  enters  the  tank  through  the 
trunk  sewers  which  are  cross  connected  with  an  overflow 
pipe  that  can  be  used  when  necessary  as  a  storm  overflow. 


48 


SEWAGE    PURIFICATION    AND    DISPOSAL 


Fig.  5 


Each  inlet  is  separately  controlled  by  a  sluice  valve 
operated  from  the  floor  above  the  tank,  so  that  sewage 
can  be  shut  off  from  either 
tank  and  crossed  over  into 
the  other,  thus  allowing  for 
cleaning  and  repairing. 
Sluice  valves  likewise  con- 
trol the  outlets  to  the  several 
septic  tanks,  so  that  sewage 
can  be  discharged  into  any, 
all  or  none  of  them.  The 
detritus  tank  is  formed  by 
the  dwarf  wall  or  weir, 
which  prevents  heavy  parti- 
cles from  passing  through 
to  the  collecting  gallery  and  from  there  to  the  septic  tanks. 
Sluice  valves  are  provided  in  the  dwarf  wall  so  the 
detritus  tanks  can  be  drained.  In  operation,  however, 
these  valves  are  kept  closed. 

Three  sets  of  removable  screens  are  provided 
in  each  screen  chamber.  The  number  of  screens 
can  be  varied,  however,  to  suit  each  installation, 
as  can  also  the  size  of  mesh  in  the  screens.  For 
extremely  large  plants  more  screens  and  larger 
mesh  screens  would  be  used  than  for  small  do- 
mestic sewage  plants.  For  ordinary  size  disposal 
|  works  treating  domestic  sewage, 

screens  of  about  i-inch  mesh  will 
be  found  satisfactory.  The  trunk 
sewers  should  be  bi-passed  around 
the  screen  chamber  and  septic 
tanks  so  the  crude  sewage  can  be 
discharged  into  the  filter  beds  or 
into  the  place  of  final  disposal 
without  being  purified.  The  bi- 
passes,  none  of  which  are  shown  in 
the  illustration,  would  have  to  be  provided  with  gate  valves 
to  control  the  flow  of  sewage  through  the  various  branches. 


L 


Pig.  6 


SEWAGE    PURIFICATION    AND    DISPOSAL 


SLUICE  GATES  AND  VALVES 

Shear  Gates — The  various  pipes,  ducts  and  flumes 
around  a  purification  works  are  valved 
so  that  the  flow  of  sewage  can  be  regu- 
lated and  controlled,  thus  making  it 
possible  to  shut  off  the  flow  entirely  or 
regulate  the  flow  to  as  small  a  stream 
as  desired.  For  this  purpose  various 
kinds  of  sluice  gates  and  gate  valves 
have  been  designed  to  meet  the  re- 
quirements of  the  various  conditions 
under  which  they  are  used.  A  simple 
shear  gate  used  at  many  small  purifi- 
cation plants  is  shown  in  Fig.  5.  This 
consists  of  a  wooden  blade,  «,  made 
wedge-shaped,  so  that  when  forced 
down  in  the  groove,  ^,  the  face  of  the 
blade  or  paddle  will  press  firmly 
against  the  face  of  the  pipe  and  cut 
off  the  flow.  This  form  of  shear  is 
sometimes  used  to  shut  off  or  regu- 
late the  flow  of  sewage  in  flumes  and  distributing  troughs 
to  filter  beds.  The  cast-iron  shear 
gate  shown  in  Fig.  6  is  an  im- 
provement over  the  wooden  pad- 
dle or  blade,  and  can  be  used  to 
control  sewage  under  high  pres- 
sures. This  shear  valve  can  be 
used  in  small  septic  tanks  to  con- 
trol the  inlet,  outflow  or  the  sludge 
pipe. 

The  sleeve,  a,  can  be  built 
into  the  masonry  or  bedded  in  con- 
crete to  make  a  tight  joint  at  this 
point.  The  gate  can  be  opened  or 
closed  by  means  of  the  handle,  #, 
which  is  notched  at  r,  to  catch  on  the  guard,  d,  and  thus 
lock  the  gate  partly  open.  The  guard  prevents  the  handle 


Fig.  7 


Pig. 


50 


SEWAGE    PURIFICATION    AND    DISPOSAL 


falling  over  to  one  side  in  the  tank,  and  the  catch,  y,  holds 
the  gate  firmly  against  the  face  of  the  sleeve  when  the 
valve  is  closed,  thus  completely  shutting  off  the  flow  of 
sewage. 

Sluice  Gates — In  large  size  purification  plants  where 


Fig.  9 


the  sewer  main  and  effluent  pipes  are  large,  a  stronger  and 
better  gate  than  the  shear  gate  is  required  to  properly 
control  the  flow  of  sewage.  For  this  purpose  sluice  gates, 
Fig.  7,  are  generally  used.  These  gates  are  bronze 
mounted,  may  be  had  with  extension  screws  and  in  round, 
rectangular  or  square  patterns.  A  square-pattern  sluice 
gate  is  shown  in  Fig.  8.  These  patterns  can  be  had  in 


SEWAGE    PURIFICATION    AND    DISPOSAL 


51 


stock  sizes  up  to  4  feet  in  diameter  for  round  sluice  gates 
and  4x4  feet  for  square  gates. 

Sluice   Gates   with   Thimble   Set   in   Concrete— There 
are  three  different  ways  of  connecting  sluice  gates  to  the 
walls  of  a  tank  or  to  the  piping  entering  a  tank.      When 
a  sluice  gate  is  to  be  set  against  a  face  of  cement 
concrete,  a  thimble,  a,  Fig.  9,  is  usually  bedded  in 
the  concrete  and  the  sluice  gate  bolted  to  the  face 
of  the  thimble. 

Sluice  Gates  Anchored  to  Masonry  —  Sluice 
gates  are  sometimes  attached  to  concrete  walls  in 
the  same  manner  that  they  usually  are  connected 
to  brick  or  masonry  walls. 

This  method  is  shown  in  Fig.  10.  Anchor  bolts, 
#,  tf,  are  embedded  in  the  wall  at  the  time  the  brick 
or  stone  is  laid,  then  when  the  wall 
is  complete  and  the  cement  set,  the 
sluice  gate  is  bolted  to  the  wall 
with  a  layer  of  cement  mortar 
packed  between  the  flange  and  the 
mason  work.  A  wooden  templet 
with  holes  bored  in  the  same  rela- 
tive positions  as  in  the  sluice  gate 
frame  is  used  to  hold  the  bolts  in 
place  while  being  bedded  in  the 
masonry. 

Sluice  Gate  Bolted  to  Pipe- 
When  sluice  gates  are  to  be  con- 
nected to  iron  pipe  they  generally 
are  bolted  to  a  flange  of  the  pipe 
as  shown  in  Fig.  n.  Instead  of 

this  method,  however,  the  sluice  gate  is  sometimes  cast 
with  a  spigot  end  so  that  the  spigot  can  be  calked  to  a  cast- 
iron  pipe  hub. 

Gate  Operating  Devices — Small  sluice  valves,  also  gate 
valves,  are  easily  operated  by  hand  without  the  aid  of 
mechanism.  In  large  plants,  however,  the  sluice  valve  or 
gate  valve,  as  the  case  might  be,  is  extremely  heavy  and 


Fig.  11 


52 


SEWAGE    PURIFICATION    AND    DISPOSAL 


Fig.  12 


sometimes  is  located  at  a  very  low  elevation.  Under  such 
conditions  a  stem  is  generally  extended  to  above  the  grade 
line  where  it  terminates  in  a  gate  house,  and  a  capstan  is 
provided  direct  connected  to  the  stem 
of  the  sluice  or  gate,  so  that  the  gate  or 
sluice  can  be  controlled  from  the  gate 
house.  Fig.  12  shows  one  form  of  capstan 
for  medium  size  gates.  This  device  is 
provided  with  a  cast-iron  hand-wheel 
which  has  three  sockets,  a,  a,  a,  cast  in  the 
hub,  into  which  capstan  bars  can  be  in- 
serted to  start  the  gate  when  closed,  or 
to  firmly  seat  the  gate  when  closing. 
After  starting  the  gate  with  the  capstan 
bars,  it  can  easily  be  raised  with  the  hand-wheel  alone. 
Large  sluice  gates,  that  are  too  heavy  to  be  easily 
operated  by  a  hand  wheel,  are  operated  by  some  modifica- 
tion of  the  geared  capstan  shown  in  Fig.  13,  or  by  electric 
motors  or  hydraulic  lifts. 

Gate   Valves — The  flow   of   sewage  through  pipes  at 
purification  works,  is  controlled  by  sluice  gates  when  the 

gates  can  be  placed  in  a 
tank  or  manhole;  when, 
however,  the  valve  must 
be  placed  on  a  pipe,  a  gate 
valve  similar  to  the  one 
shown  in  Fig.  14  is  gener- 
ally used.  Valves  of  this 
type  can  be  had  in  any 
size  up  to  10  feet  in  diam- 
eter and  with  or  without 
spur  gear  for  opening  and 
closing  the  gate.  The 
smaller  sizes  are  not  pro- 
vided with  gears,  as  the 

gate  can  be  operated  direct  from  a  hand  wheel  or  by  means 
of  a  key  with  cross  arms  operated  by  two  or  four  men. 
A  gate  of  this  description  can  be  located  in  a  valve  pit  and 


LJ 


SEWAGE    PURIFICATION    AND    DISPOSAL 


53 


operated  by  means  of  a  cross-arm  key,  or  the  stem,  a,  can 
be  extended  to  a  gate  house  above,  from  which  place  it  can 
be  operated  by  means  of  a  capstan,  an  electric  motor  or  a 
hydraulic  motor. 


DOSING  APPARATUS 

Miller  Automatic  Siphon — A  Miller  siphon  of  slightly 
different  design  from  the  one 
shown  in  connection  with  the  sep- 
tic tank,  is  illustrated  in  Fig.  15. 
The  action  of  this  siphon  is  as 
follows :  Assuming  that  the  trap 
is  full  of  water,  then,  as  sewage 
flows  into  the  tank  it  gradually 
rises  above  the  bottom  of  the  bell, 
thus  confining  the  air  between  the 
mouth  of  the  bell  and  the  surface 
of  the  water  in  the  trap.  As  the 
head  of  water  in  the  tank  in- 
creases, it  compresses  the  air  in 
the  long  leg  of  the  trap,  thus 
gradually  forcing  out  the  water 
until  a  point  is  reached  when  the 
air  is  about  to  escape  around  the 
lower  bend. 

The  difference  in  water  level 
in  the  two  legs  of  the  trap  is  at 
all  times  equal  to  the  head  of 
water  above  the  level  of  water  in 
the  bell ;  consequently,  when  the 
water  in  the  long  leg  of  the  trap  is  depressed  to  the  point 
shown  in  the  illustration,  the  siphon  is  about  ready  to  oper- 
ate, and  any  further  increase  in  the  depth  of  water  in  the 
tank  will  force  the  air  around^the  lower  bend,  and  in  its 
upward  rush  the  air  will  carry  with  it  some  of  the  water 
in  the  short  leg  of  the  trap,  thus  destroying  the  equili- 
brium of  the  two  columns  of  water  and  bringing  the  siphon 


Fig.  14 


54 


SEWAGE    PURIFICATION    AND    DISPOSAL 


immediately  into  full  action.  The  water  is  thus  drawn  out 
of  the  tank  to  the  bottom  of  the  bell,  the  siphon  broken  by 
the  admission  of  air  through  the  snift  hole,  #,  and  the 
siphon  is  again  ready  for  action. 

The  depth  of  sewage  that  will  accumulate  in  a  tank, 
before  the  siphon  is  brought  into  action,  depends  on  the 
length  of  the  short  leg  of  the  trap.  The  head,  b,  from  the 
level  of  liquid  within  the  bell  to  the  surface  of  liquid  in  the 


Fig.  15 


tank  will  equal  the  distance,  <:,  from  the  bend  of  the  trap 
to  the  surface  of  the  outlet  pipe.  In  calculating  the 
quality  of  liquid  that  will  be  discharged  from  the  tank  at 
each  operation  of  the  siphon,  the  head  or  depth,  d,  must 
be  assumed,  as  the  liquid  will  be  lowered  at  each  discharge 
to  the  top  of  the  snift  hole. 

To  find  the  quantity  of  the  discharge,  therefore,  or 
to  calculate  the  size  of  the  tank  required  to  discharge  a 
given  quantity  of  sewage,  multiply  the  area  of  the  tank 
in  square  feet  by  the  head,  d,  in  feet.  The  product  will  be 
the  quantity  in  cubic  feet  discharged  at  each  operation  of 
the  siphon.  The  illustration  shows  the  outlet  leg  of  the 
trap  discharging  into  a  pipe  drain.  Instead  of  this 


SEWAGE    PURIFICATION    AND    DISPOSAL 


55 


arrangement,  however,  the  siphon  can  discharge  into  a 
special  dosing  tank  connected  to  a  drain,  or  into  an  open 
flume.  The  only  requirement  in  any  case  is  that  the  over- 
flow edge  of  the  short  leg  project  into  the  tank,  pipe,  or 
flume,  so  that  the  water  carried  out  by  the  air  can  instan- 
taneously escape.  If  the  discharge  mouth  were  formed  as 
an  ordinary  bend,  the  siphon  would  not  operate,  because 
the  heaved-up  water  would  have  no  means  of  instantaneous 


Fig.  16 


escape,  and  therefore  the  equilibrium  of  the  two  columns 
of  water  would  not  be  sufficiently  upset. 

A  Miller  automatic  siphon,  similar  to  the  one  shown  in 
the  illustration  of  a  septic  tank,  is  shown  in  section  in  Fig. 
1 6.  This  siphon  differs  from  the  one  shown  in  Fig.  15, 
principally  in  having  a  vent  pipe  «,  instead  of  a  snift  hole 
in  the  bell.  By  using  a  vent  pipe  a  greater  depth  of 
sewage  can  be  accumulated  in  the  dosing  chamber  before 
the  siphon  is  brought  into  action.  This  is  due  to  the  fact 
that  the  air  in  the  long  leg  of  the  siphon  can  escape  until 
the  sewage  in  the  tank  rises  high  enough  to  seal  the  mouth 


56 


SEWAGE    PURIFICATION    AND    DISPOSAL 


of  the  vent  pipe,  which,  while  sufficiently  large  to  permit 
the  escape  of  air  from  the  confined  space,  is  not  large 
enough  to  break  the  siphonic  action,  when  the  mouth  is 
unsealed,  before  the  sewage  is  lowered  to  the  mouth  of  the 
bell.  This  type  of  siphon  or  some  of  its  modifications  is 
the  one  most  extensively  used  in  purification  plants. 

Capacity  of  Siphons — The  size  of  siphon  required  to 
empty  a  sewage  tank  depends  somewhat  upon  the  place  of 
discharge.  For  instance,  if  the  effluent  is  to  be  discharged 
intermittently  into  a  stream,  lake,  ravine,  or  into  tide  water, 
the  length  of  time  required  to  empty  the  tank  is  of  less 
importance  than  when  the  effluent  is  to  be  discharged  onto 
a  filter  bed  or  into  contact  beds.  When  sewage  is  dis- 
charged direct  into  water,  a  slow  discharge  will  be  found 
conducive  to  greater  dilution,  and  under  such  conditions  a 
small  siphon  no  doubt  would  be  preferable.  In  the  case 

TABLE  V — CAPACITY  OK  MILLER  AUTOMATIC  SIPHONS 


Diameter  of  Trap 

Rate  of  Discharge  per  Second 

in  Inches 

Cubic  Feet 

U.  S.  Gallons 

3 

.2 

w 

5 

.65 

5 

6 

1.00 

7^ 

8 

2.00 

15 

of  sand  filters  on  the  other  hand,  the  beds  should  be  flooded 
in  a  comparatively  short  period  of  time  to  bring  all  parts 
of  the  filter  into  service  simultaneously;  and  in  the  case 
of  contact  beds  the  tank  should  be  filled  in  a  certain  period 
of  time,  usually  two  hours. 

When  designing  a  purification  plant,  the  places  of 
disposal  should  be  considered  and  the  quantity  of  liquid 
to  be  discharged  in  a  given  time  calculated;  then,  if  a 
Miller  siphon  is  to  be  used,  the  required  size  can  be  found 
in  the  following  table  of  manufacturers'  ratings,  or  the 
size  can  be  calculated  by  the  formula  given  on  page  85. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


57 


Rhoads=niller  Automatic  Siphon  —  An  automatic 
siphon  of  the  Rhoads-Miller  type  is  shown  in  Fig.  17. 
The  action  of  this  siphon  depends  on  the  sudden  releasing 
of  the  compressed  air  confined  in  the  long  leg  of  the  trap, 
between  the  water  in  the  bell,  a,  and  the  water  in  the  deep- 
seal  trap,  b.  Assuming  that  the  deep-seal  trap,  £,  and  the 
blow-off  trap,  c,  are  sealed  with  water,  the  operation  of 
the  siphon  will  be  as  follows:  Sewage  will  flow  into  the 
tank  until  it  covers  the  mouth  of  the  bell,  #,  and  the  outlet, 


Fig.  17 


d,  to  the  vent  pipe.  The  air  confined  in  the  space  between 
the  deep-seal  trap  and  the  bell  will  then  become  compressed 
in  proportion  to  the  head  of  water  in  the  tank,  until  the 
pressure  becomes  great  enough  to  force  the  water  in  the 
blow-off  trap  down  to  the  bend  or  dip,  £,  that  forms  the  seal. 
Any  greater  head  of  water  will  then  force  the  water  out  of 
the  blow-off  trap,  thus  releasing  the  air  from  the  confined 
space  which  immediately  fills  with  water,  thereby  bringing 
the  siphon  into  action.  When  the  sewage  has  been  low- 
ered to  the  mouth  of  the  pipe,  </,  air  will  enter  through 
this  pipe  and  break  the  siphonage. 


58 


SEWAGE    PURIFICATION    AND    DISPOSAL 


Plural  Alternating  Siphons — Alternating  siphons  are 
used  to  discharge  the  effluent  from  a  septic  tank  onto  two 
or  more  filter  beds  in  rotation,  so  that,  between  dosings, 
each  bed  will  have  a  period  of  rest  during  which  the 
interstices  of  the  sand  or  other  filtering  medium  can  fill 
with  air  and  thus  provide  oxygen  for  the  reduction  of  the 
organic  matter  that  is  contained  in  the  filter. 

A  plural  alternating  siphon  of  the  Miller  type  is 
shown  in  Fig.  18. 


Fig.  18 

This  apparatus  is  designed  to  automatically  discharge 
the  contents  of  a  dosing  chamber  onto  several  filter  beds 
in  rotation.  The  operation  of  the  apparatus  is  based  on 
the  principle  that,  when  two  or  more  siphons  are  set  at  the 
same  elevation  in  a  tank,  that  siphon  which  contains  the 
least  water  will  discharge  first;  and,  when  installing  an 
alternating  siphon,  the  traps  are  cross-connected  in  such  a 
manner  that  they  all,  with  the  exception  of  the  one  next 
to  discharge,  will  refill  at  every  discharge  of  sewage  from 
the  tank. 

The  alternating  siphon  is  operated  as  follows  :  Assum- 
ing that  the  main  traps,  a-a* ',  and  the  blow-off  traps,  b-ft ', 


SEWAGE    PURIFICATION    AND    DISPOSAL  59 

are  sealed,  and  the  wells  in  the  center  filled  with  sewage, 
then  when  sewage  rises  above  the  mouth  of  the  pipes,  c-c1, 
the  air  in  the  traps  will  become  confined  and  will  be  com- 
pressed in  proportion  to  the  rise  of  sewage  in  the  tank.  As 
the  air  becomes  compressed  it  forces  the  sewage  down  in 
all  the  traps,  as  shown  in  the  section,  until  air  is  about 
to  escape  around  the  dip  of  the  trap  which  holds  the  least 
sewage.  If,  in  the  present  example,  the  siphon  shown  in 
section  were  the  first  to  discharge,  sewage  in  flowing  past 
the  mouths  of  the  pipes,  </,  e  and  /,  would  siphon  sewage 
out  of  the  two  wells  to  which  the  pipes  are  connected,  while 
the  other  two  wells  would  remain  full.  These  full  wells 
would  then  fill  the  two  traps  to  which  they  are  connected, 
while  the  trap  shown  in  section  would  immediately  refill 
after  siphoning,  thus  leaving  the  seal  of  the  remaining 
trap  weakened  so  it  would  be  the  next  to  operate.  Any 
number  of  siphons  can  be  connected  up  to  operate  automati- 
cally in  rotation  and  they  can  be  set  in  rows  or  in  a  circle. 
It  is  not  necessary,  however,  to  provide  wells  and  cross  con- 
nect the  traps  when  only  two  siphons  are  in  battery,  and 
when  only  three  Miller  siphons  are  to  alternate,  the  wells 
can  be  omitted  although  the  traps  must  be  cross-connected 
to  one  another. 

Barbour  Rotation  Dosing  Apparatus — A  rotation  dosing 
apparatus,  designed  by  F.  A.  Barbour,  of  Boston,  Mass., 
and  successfully  used  in  a  number  of  plants,  is  shown  in 
Fig.  19.  In  this  apparatus  a  Miller  siphon  is  used,  which 
discharges  into  a  distributing  stand-pipe  controlled  by  a 
revolving  gate.  Briefly,  the  operation  of  the  apparatus  is 
as  follows  :  Assuming  that  the  main  trap,  a,  and  the 
blow-off  trap,  #,  are  filled  with  sewage,  which  has  raised 
above  the  mouth  of  the  bell,  r,  and  of  the  vent  pipe,  d,  thus 
confining  and  compressing  the  air  in  the  space,  ^,  and  blow- 
off  pipe,  y,  then  as  sewage  flows  into  the  dosing  chamber  it 
raises  the  float,  g,  to  the  level  shown  in  the  illustration,  thus 
by  means  of  the  ratchet  and  pawl,  //,  and  bevelled  gear,  z, 
turning  the  rotary  gate,  /,  in  the  stand-pipe  until  the  open- 
ing is  opposite  the  outlet  to  the  next  field  to  be  flooded.  It 


60 


SEWAGE    PURIFICATION    AND    DISPOSAL 


is  obvious  that  the  distance  the  float,  g,  is  permitted  to  rise 
will  depend  on  the  number  of  outlets  in  the  stand-pipe,  a 
greater  distance  being  necessary  when  only  two  or  three 
openings  are  provided  than  when  there  are  outlets  to  five 
or  six  filter  beds.  A  stop  is  therefore  provided,  so  that  the 
float  cannot  rise  higher  than  a  certain  level  while  the  ratchet 
permits  the  float  to  return  to  its  place  at  the  bottom  of 
the  tank  without  disturbing  the  gate,/ 


Fig.  19 


In  order  to  make  the  discharges  automatic,  and  so  they 
can  be  regulated  to  any  desired  height  of  sewage  in  the 
tank,  a  second  float,  k,  is  provided  and  is  attached  to  a  rod 
which  works  in  guides.  Above  this  float  and  attached  to  the 
rod  is  a  tappet  arm,  /,  which,  when  the  float  reaches  a  cer- 
tain level  in  the  tank,  trips  the  lever,  mt  which  in  turn  raises 
the  weighted  arm,  «,  that  closes  the  escape  valve,  o.  Air  is 
thus  released  from  the  space,  ^,  which  immediately  fills  with 
water  and  the  siphon  is  brought  into  action.  By  having  an 
adjustable  float,  £,  the  siphon  can  be  made  to  operate  at 


SEWAGE    PURIFICATION    AND    DISPOSAL  61 

different  levels,  thus  making  it  possible  to  change  the  size 
of  the  dose  applied  to  the  beds,  a  condition  which  is  some- 
times desirable,  under  different  weather  conditions,  it  being 
understood  that  in  summer  small  doses  more  frequently 
applied  at  high  rates,  give  the  best  results,  while,  during 
winter  in  cold  climates,  a  larger  dose  is  required  to  prevent 
freezing  of  the  beds. 

The  gate,  /,  can  be  operated  by  hand,  by  turning  it  by 
means  of  the  wheel,  /.  A  sluice  gate,  ^,  with  its  handle,  r, 
projecting  above  the  stand-pipe,  is  provided  so  sewage  can 
be  drained  from  the  tank  without  passing  through  the 
siphon.  A  separator,  s,  is  placed  in  the  air  pipe  to  prevent 
any  solid  matter  being  carried  to  and  interfering  with  the 
operation  of  the  escape  valve,  o.  The  blow-off  trap,  £,  is 
provided  to  insure  the  discharge  of  the  larger  siphon  at  a 
predetermined  head,  regardless  of  whether  or  not  the  auxil- 
iary float  and  escape  valve  on  the  air  pipe  work. 


INTERMITTENT  FILTRATION  OF  SEWAGE 

Principles  of  Intermittent  Filtration — If  organic  matter 
be  deposited  on  the  surface  of  the  ground,  or  buried  in  the 
upper  layers  of  the  earth,  certain  changes  will  take  place  in 
the  organic  matter,  which  in  a  short  time  will  disappear 
leaving  only  a  residue  of  mineral  ash  that  is  soft  and  greasy 
to  the  touch,  and  resembles  in  its  composition  the  humus  of 
loamy  soil.  The  changes  that  take  place  in  the  organic  mat- 
ter are  brought  about  by  micro-organisms  with  which  the 
upper  layers  of  earth  fairly  teem,  in  many  cases  numbering 
millions  of  bacteria  to  each  gram  of  soil.  Most  of  the  micro- 
organisms of  the  soil  are  aerobic  and  are  found  in  greatest 
number  near  the  surface,  in  the  first  half-inch  layer.  From 
this  point  they  decrease  in  number  with  the  depth,  until  at 
a  point  five  feet  from  the  surface  they  are  so  few,  compara- 
tively, that  the  purification  effected  below  that  depth  is  but 
slight.  Tests  made  by  the  Massachusetts  Board  of  Health 
to  determine  the  comparative  number  of  bacteria  in  a  gram 


62  SEWAGE    PURIFICATION    AND    DISPOSAL 

of  sand  at  different  depths  in  a  filter  bed  give  results  shown 
in  Table  VI. 

The  same  agencies  that  operate  to  reduce  organic  mat- 
ter in  the  upper  layers  of  soil  are  equally  active  in  a  filter 
bed  made  of  sand.  After  a  sand  filter  has  been  ripened  by 
intermittent  applications  of  sewage,  for  a  certain  length 
of  time,  if  sewage  is  then  applied  to  it  intermittently  at 
certain  periods  of  time,  and  sufficient  intervals  are  allowed 
between  the  applications  for  the  interstices  of  the  sand  to 
be  drained  of  water  and  thoroughly  aerated,  it  will  be  found 

TABLE  VI — BACTERIA  FOUND  AT  DIFFERENT   DEPTHS   IN  FILTER  BEDS 


Distance  from  Surface 

Number  of  Bacteria  per  Gram  of  Soil 

0      to     l/2  inch 

1,760,000 

YZ  to    >/  inch 

105,000  • 

IX  to  I/?  inch 

207,200 

2  inches 

60,200 

3  inches 

111,300 

5  inches 

63,400 

8  inches 

30,700 

12  inches 

34,100 

19  inches 

12,300 

60  inches 

4,100 

that  the  organic  matter  in  the  sewage  will  disappear  and 
that  the  efHuent  from  the  filter  will  be  clear,  odorless  and 
sparkling.  The  changes  which  take  place  in  a  filter  bed 
are  not  due  to  the  mere  mechanical  process  of  straining, 
but  are  the  result  of  a  biological  process  similar  to  that 
in  a  garden  soil.  If  the  grains  of  sand  are  examined  micro- 
scopically it  will  be  found  that  they  are  enveloped  in  a  thin 
film  of  gelatinous  material  in  which  are  entangled  the  re- 
ducing micro-organisms.  This  gelatinous  film  is  pervious 
to  water  and  under  its  influence  swells,  so  that  when  sewage 
is  applied  to  a  filter  bed,  the  gelatinous  films  that  cover  the 
grains  become  saturated  with  water  and,  by  swelling,  fill 
the  interstices  between  the  sand  grains,  thus  interposing  a 
barrier  against  the  passage  of  organic  matter  without 
greatly  retarding  the  flow  of  water.  The  bacteria  in  the 
gelatinous  film  attack  the  organic  matter  intercepted  by 


SEWAGE    PURIFICATION    AND    DISPOSAL  63 

the  film  and  quickly  convert  it  to  useful  nitrites  and 
nitrates. 

It  is  now  generally  understood  that  the  mechanical 
separation  of  any  part  of  the  sewage  by  straining  through 
sand  is  but  an  incident,  which  under  some  conditions  may 
favorably  modify  the  result ;  but  the  essential  condition  is 
a  slow  movement  of  a  thin  film  of  liquid  over  the  surface 
of  the  particles,  and  sufficient  voids  between  the  particles 
to  allow  air  to  be  continually  in  contact  with  the  films  of 
liquid. 

If  the  action  of  a  filter  were  simply  a  straining  process, 
the  surface  of  the  filter  and  the  interstices  between  the  sand 
grains  would  soon  be  covered  with  a  layer  of  sludge  that 
would  clog  the  filter.  As  a  matter  of  fact,  when  a  filter  is 
not  overdosed,  no  such  clogging  occurs ;  on  the  contrary, 
sewage  containing  sufficient  organic  matter  to  fill  the  voids 
of  the  sand  bed  many  times  over  has  been  applied  to  filters 
daily  for  years,  with  the  result  that  in  every  case  all 
organic  matter  has  been  reduced,  and  the  effluent  from 
the  filters  has  been  clear,  sparkling,  odorless,  and  pure  as 
many  drinking  waters,  while  with  the  exception  of  a  small 
amount  of  cellulose  material,  no  organic  matter  can  be 
found  on  the  surface  of  the  filters  or  in  the  interstices  of 
the  sand. 

Furthermore,  in  filters  that  are  constructed  of  coarse 
material,  for  instance  of  crushed  stones  about  the  size  of 
a  walnut,  a  purification  of  the  sewage  is  effected  although 
the  interstices  between  the  stones  are  too  large  for  the 
filter  to  act  as  a  strainer. 

It  is  absolutely  necessary  for  the  successful  operation 
of  a  filter  that  the  application  of  sewage  be  intermittent, 
and  that  sufficient  time  be  allowed  between  doses  for  the 
organic  matter  to  be  entirely  reduced;  otherwise  the  sur- 
face of  the  filter  bed  will  become  clogged  with  organic 
matter,  which,  in  some  instances,  becomes  very  thick  and 
so  interwoven  that  it  can  be  rolled  up  like  a  mat,  while  in 
other  cases  the  organic  matter  becomes  caked  on  the 
surface. 


64  SEWAGE    PURIFICATION    AND    DISPOSAL 

The  intervals  between  which  filter  beds  may  be  flooded 
depends  to  a  great  extent  on  the  strength  and  freshness 
of  the  sewage  and  on  the  size  of  the  filtering  interstices. 
Less  time  is  required  between  applications  when  treating 
a  weak  domestic  sewage  from  a  septic  tank  on  a  coarse 
sprinkling  filter,  than  would  be  required  between  applica- 
tions on  an  intermittent  sand  filter  when  treating  a  strong 
sewage  that  is  comparatively  fresh.  For  an  intermittent 
sand  filter,  treating  average  sewage,  in  moderate  climates, 
it  seems  best  to  divide  the  daily  dose  into  four  portions 
to  be  applied  at  equal  intervals  of  six  hours.  During 
winter  weather,  however,  in  cold  climates,  it  is  better  to 
apply  larger  doses  at  less  frequent  intervals  to  prevent 
freezing  of  the  beds. 

In  sewage  purification  by  intermittent  filtration,  two 
classes  of  bacteria  are  brought  into  action.  When  the  filter 
beds  are  flooded  with  sewage,  anaerobic  bacteria  become 
active  in  breaking  down,  gasifying  and  liquefying  the  solid 
matter,  after  which,  when  the  water  has  drained  from  the 
bed,  aerobic  bacteria  complete  the  process  by  nitrifying  the 
ammonia  compounds. 

EXAflPLE  OF  AN   INTERMITTENT  FILTER 

An  intermittent  filter  is  simply  a  bed  of  sand,  or  other 
porous  material,  suitably  underdrained  to  carry  off  the 
liquid  that  percolates  through  the  filter  bed.  Where  filter 
beds  are  artificially  constructed  they  are  generally  pro- 
vided with  underdrains  of  tile,  brick  or  stone.  In  some 
localities,  however,  sand  in  its  natural  bed  is  available,  and 
when  such  sand  fields  have  a  low  water  table  they  may  be 
successfully  used  without  pipe  underdrains,  the  liquid  that 
percolates  through  the  sand  being  allowed  to  follow  the 
natural  course  of  the  ground  water.  It  would  seem  a  better 
practice,  however,  to  provide  underdrains  even  when  the 
filter  beds  are  made  in  natural  deposits  of  sand.  An  artificial 
filter  bed  is  shown  in  Fig.  20.  In  this  illustration  the  em- 
bankments and  bottom  of  the  filter  are  puddled  with  clay, 
and  the  slopes  and  tops  of  the  embankments  are  sodded 


SEWAGE    PURIFICATION    AND    DISPOSAL 


65 


with  grass.  The  underdrains,  a,  are  usually  made  of  ordi- 
nary salt-glazed  sewer  pipe  with  the  bottom  portion  of  the 
hub  broken  or  cut  straight  across  so  the  lengths  will  have 
a  firm  bearing  on  the  bottom  of  the  filter.  Around  the 
underdrains  the  filter  is  filled  to  a  depth  of  several  inches 
with  broken  stone  that  will  pass  through  a  2 -inch  ring; 
above  the  broken  stone  are  placed  layers  of  graduated  sizes 
of  stone  and  gravel  to  pro  vide  a  base  for  the  bed  of  sand, 
£,  which  usually  is  about  5  feet  in  depth.  The  graduated 
sizes  of  stone  and  gravel  are  interposed  to  prevent  sand 
from  falling  down  and  clogging  the  underdrains,  or  filling 
the  interstices  of  the  stones  so  the  effluent  would  have 
to  force  its  way  laterally  to  the  drains. 


Fig.  20 

On  top  of  the  sand  bed,  a  system  of  distributing 
troughs,  ^,  is  provided  to  distribute  as  evenly  as  possible 
the  flow  of  sewage  to  all  parts  of  the  filter,  so  that  each 
section  will  have  imposed  on  it  an  equal  share  of  the  bur- 
den. The  surface  of  the  filter  bed  is  usually  protected  at 
the  end  of  the  troughs,  at  d,  by  paving  with  stones,  flags 
or  cement  blocks,  to  prevent  the  sand  being  washed  away ; 
and  sluice  gates  are  provided  as  at  e  to  control  the  flow 
of  sewage  to  each  bed  unless  each  bed  is  flooded  direct 
from  a  dosing  tank  through  a  separate  and  independent 
pipe,  in  which  case  the  sluice  gates  are  omitted.  Where 
inlet  pipes  pierce  the  embankment,  as  shown  at  ^,  the 
slope  should  be  protected  by  a  bulkhead  of  brick,  stone,  or 


66  SEWAGE    PURIFICATION    AND    DISPOSAL 

concrete,  and  the  surface  of  the  filter  bed,  at  this  point, 
should  be  paved  with  the  same  material.  The  cover  to 
the  manhole  where  the  main  effluent  pipe  from  all  the 
beds,  and  the  main  drain,  #,  from  the  bed  illustrated,  inter- 
sect, is  shown  at/.  It  is  a  difficult  matter  to  lay  down  rules 
for  the  construction  of  filter  beds  that  will  be  applicable  to 
all  cases.  Illustrations  showing  the  principles  involved  can 
be  given,  and  the  originality  of  the  designer  must  then 
adapt  to  his  use  the  method  best  suited  to  his  purpose. 


DETAILS  OF  INTERMITTENT  FILTERS 

Filter  Basin — The  materials  of  which  filter  basins  are 
made  depend  much  on  the  availability  of  various  building 
materials  in  the  locality  where  the  purification  plant  is  to  bet 


CROSS   SECTION  THRO'  FILTER  BED. 
Fig.  21 

constructed,  also  on  the  type  of  filter  bed.  When  sprink- 
ling, or  percolating,  filters  are  to  be  constructed,  an  imper- 
vious flooring  is  provided  and  sometimes  the  system  of 
underdrains  and  the  filtering  materials  are  placed  on  the 
flooring  without  being  confined  by  walls  or  embankments. 
In  the  construction  of  intermittent  sand  filters,  however, 
the  filtering  material  usually  is  enclosed  with  a  wall  or  with 
an  earthen  embankment  that  is  impervious  to  water.  It 
logically  follows  that  in  a  locality  where  clay  is  plentiful, 
and  sand,  stone  and  cement  expensive,  puddled  clay  bot- 
toms and  embankments  would  be  more  economical  than 
masonry  walls  and  floors.  Under  such  conditions,  and  in 
view  of  the  further  fact  that  a  puddled  clay  basin  will 
prove  equally  serviceable,  this  form  of  construction  would 


SEWAGE    PURIFICATION    AND    DISPOSAL 


67 


doubtless  be  used;  whereas  in  a  locality  where  cement  and 
sand  are  cheap,  and  clay  expensive,  a  concrete  flooring  and 
enclosure  would  be  preferable. 

In  laying  a  flooring  and  embankment  of  clay,  greater 
solidity  is  obtained  by  laying  alternate  courses  of  clay  and 
gravel  than  would  be  obtained  by  making  the  floor  and  em- 
bankments entirely  of  clay,  while  at  the  same  time  an 
equally  tight  basin  is  constructed. 

A  section  of  an  intermittent  filter  showing  the  em- 
bankment, underdrains,  stone,  gravel,  sand  and  distributing 


b75??2  :•:•:.'  •••::  ;^-.  .x-o  ••.••-•  :-:-V ••-;;.••.'• 
<Lji=  ^ '.' .  :'•:.': "  • :/•  j  -'-.  Y;V  •  •'  -'.3  an  pi  •:-,-.  \ 


SECTION    THRO'  GATE  VALVES. 

Fig.  22 

sluices  is  illustrated  in  Fig.  21.  The  embankments  in 
this  illustration  are  shown  with  a  slope  of  i  to  i.  In 
some  basins,  however,  the  embankments  are  given  less 
of  a  slope,  being  laid  at  from  i^  to  2  on  the  horizontal  to 
i  on  the  rise.  The  manner  of  building  the  embankments 
by  rolling  clay  and  gravel  in  alternate  layers,  the  puddling 
of  the  basin  with  clay,  the  top  dressing  of  loam,  and  the 
sodding  of  the  embankments,  are  all  clearly  indicated  in 
the  illustration. 

Where  a  number  of  filter  beds  are  laid  out  together, 
however,  at  large  purification  works,  the  clay  and  gravel 
embankments  are  sometimes  omitted  and  ordinary  soil 
heaped  up  to  form  the  basins ;  often  when  the  soil  is  of  a 
porous  nature,  puddling  is  also  dispensed  with  and  the 


68 


SEWAGE    PURIFICATION    AND    DISPOSAL 


underdrains  laid  on  undisturbed  earth.  In  fact,  many 
modifications  of  the  filter  shown  in  the  illustration  are  em- 
ployed when  designing  filter  beds,  simplicity  of  design  and 
economy  of  construction  being  the  object  desired. 

A  section  through  the  gate  or  sluice  valves  controlling 
the  flow  onto  filter  beds  is  shown  in  Fig.  22,  and  a  section 
through  the  manholes  located  at  the  intersection  of  the 
main  drain  from  each  filter  bed  and  the  main  effluent  pipe 
is  shown  in  Fig.  23. 


FILTER  UNDERDRAINS 

Systems   of    Underdrains — A    system    of    underdrains 


THRO'  MANHOLE. 
Fig.  23 

either  for  an  intermittent  filter  or  for  a  sprinkling  filter 
should  be  so  proportioned  that  the  frictional  resistance  will 
not  cause  unequal  rates  of  filtration  in  different  parts  of  the 
bed.  Equal  rates  of  filtration  can  be  maintained  by  pro- 
portioning the  size  of  the  underdrains  to  the  area  they  are 
to  drain  and  the  amount  of  liquid  they  are  to  conduct  in  a 
given  time.  As  the  rate  at  which  filter  beds  are  operated 
vary  under  different  conditions,  it  is  impossible  to  state 
a  size  of  pipe  to  use  for  a  unit  area  of  filter  bed.  The 
size  can  readily  be  ascertained,  however,  by  multiplying 
the  area  of  filter  bed  to  be  drained  by  the  proposed  rate 


SEWAGE    PURIFICATION    AND    DISPOSAL  69 

of  filtration,  then  calculating  the  size  of  pipe  that  will 
safely  conduct  that  quantity  of  water  when  running  half 
full.  The  advantages  of  having  drain  pipes  that  are  only 
half  filled  by  the  effluent  from  the  surfaces  drained  are 
three-fold.  In  the  first  place,  the  frictional  resistance  is 
reduced  to  the  minimum;  in  the  second  place,  provision  is 
made  for  a  greater  rate  of  filtration  should  a  different  pre- 
liminary treatment  be  subsequently  devised  that  will  effect 
a  more  complete  purification  of  the  sewage,  thus  permitting 
a  greater  rate  of  filtration ;  and  lastly,  that  portion  of  the 
underdrains  which  is  not  filled  with  liquid  will  act  as  an 
air  inlet  to  the  bottom  of  the  filter  bed,  thus  providing  a 
supply  of  oxygen  for  the  aerobic  nitrifying  bacteria  to  com- 
plete the  process  of  nitrification. 

The  size  of  drain  required  to  conduct  a  certain  quantity 
of  water  when  running  half  full  can  be  determined  by  the 
formula : 


d=.234 


h 
In  which  d=diameter  of  pipe  in  feet 

q—  cubic  feet  per  second  to  be  delivered 
l=length  of  pipe  in  feet 
h=head  in  feet 

EXAMPLE  —  What  diameter  of  pipe  will  be  required  to  drain  a 
section  of  filter  bed  containing  2,000  square  feet  of  surface  when  the 
rate  of  filtration  is  208,333  gallons  per  acre  per  hour;  the  drain  being 
100  feet  long  and  laid  at  a  grade  of  one  foot  in  100  feet  ? 

SOLUTION  —  An  acre  contains  43,  560  square  feet  of  surface,  or  about 
22  times  the  area  contained  in  the  2,000  square  feet  of  filter  bed  to  be 
drained;  consequently  if  the  filter  be  operated  at  the  rate  of  208,333 
gallons  per  hour,  the  quantity  of  sewage  filtered  by  2,000  square  feet 
would  equal  ^  of  208,333=9,469  U.  S.  gallons  per  hour.  9,469  U.  S. 
gallons  per  hour  equals  .  347  cubic  feet  per  second. 

Substituting  the  values  in  the  formula, 


d  =  .2344/  2x-3472xl°°=.234.|/  24=.442  feet=5.3  inches.    Answer. 

It  is  good  practice  to  always  calculate  the  size  of  pipe 
required  to  drain  a  filter  bed.     Much  labor  can  be  saved, 


70  SEWAGE    PURIFICATION    AND    DISPOSAL 

however,  and  equally  good  results  obtained  in  small  plants 
by  obtaining  the  size  from  Table  VII.  To  use  this  table, 
calculate  the  quantity  of  fluid  in  cubic  feet  that  must  be 
removed  per  minute,  double  the  quantity  and  find  in  the 
table  the  size  of  pipe  corresponding  to  the  grade  that  will 
safely  conduct  that  quantity  of  water.  The  quantity  of 
fluid  is  doubled  so  the  size  of  pipe  used  will  safely  care  for 
the  effluent  when  running  only  about  half  full. 

EXAMPLE — What  size  pipe  will  be  required  to  underdrain  a  section 
of  filter  bed  15x60  feet  when  the  rate  of  filtration  is  300,000  U.  S.  gal- 
lons per  hour,  and  the  drain  is  laid  at  a  grade  of  1  to  100  feet? 

SOLUTION — 15x60=900  square  feet=^T  of  an  acre;  ^  of  800,000 
U.  S.  gallons:=6,489  gallons  per  hour  equals  14.4  cubic  feet  of  fluid  to  be 
removed  per  minute. 

To  find  the  size  of  drain  pipe  that  when  running  half 
full  will  conduct  that  quantity  of  liquid,  multiply  14.4  by  2, 
which  gives  28.8,  and  find  in  Table  VII  (on  following  page) 
the  size  of  pipe  laid  at  a  grade  of  i  to  100  that  will  conduct 
that  quantity  of  water.  At  the  bottom  of  the  column 
headed  5  inches,  will  be  found  that  a  5 -inch  pipe  laid  at 
a  grade  of  i  foot  to  100  feet  will  discharge  25.92  cubic  feet 
per  minute,  and  as  that  size  of  pipe  comes  the  nearest  to 
handling  the  required  amount  of  water,  when  laid  at  the 
desired  grade,  it  should  be  used. 

If  the  drain  be  laid  at  a  greater  incline  than  i  foot  in 
ioo  feet,  a  smaller  size  of  pipe  can  be  used.  For  instance, 
by  a  reference  to  Table  VII  it  will  be  seen  that  a  4-inch 
drain  laid  at  a  grade  of  i  foot  in  30  feet  has  a  capacity  equal 
to  a  5 -inch  drain  pipe  laid  at  a  grade  of  i  foot  in  ioo  feet. 

In  proportioning  an  underdrainage  system,  the  branch 
pipes  are  not  reduced  in  size  in  proportion  to  their  distance 
from  the  effluent  outlet,  but  are  extended  in  full  size  from 
the  main  collector  to  the  end  of  the  drain.  Ordinarily  there 
are  but  two  sizes  of  pipe  used  for  underdrains  in  any  filter 
bed.  The  main  conduit,  which  is  proportioned  to  conduct 
the  flow  from  the  entire  filter,  is  extended  in  full  size 
across  the  filter,  and  the  laterals  or  branch  drains  are  pro- 
portioned to  care  for  the  flow  from  their  respective  drainage. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


71 


TABLE  VII — CAPACITY  OF  DRAINS 

Velocity    in    feet    per    minute    (as   determined    by    the    formula 
v=3,000  /i/yXd)   and    discharge    in  cubic    feet  per  minute  (by  the 

formula  Q=V  A)  of  drains  laid  at  different  grades  when  running  fulL 
In  which  V= velocity  in  feet  per  minute 
A=area  of  pipe  in  feet 
h=head  in  feet 
1= length  of  the  pipe  in  feet 
d= diameter  of  the  pipe  in  feet 


Diameter 

2  Inches 

2%  Inches 

3  Inches 

4  Inches 

5  Inches 

6  Inches 

— 

|S 

3" 

U    jj 

£ 

£| 

c 

5| 

„  .c 

S 

^c 

3| 

|| 

y 

3| 

^.S 

2*5 

&s 

Sf  «> 

Slu 

Mu 

£S 

bjcjj 

j?a 

S)u 

k"  « 

B  u 

2  D. 

H 

05  O. 

c  *. 

1? 

^  a 

rt  o. 

0    u 

J  °* 

fc 

•>  ^ 

"%  « 

?  o. 

Is 

?  0. 

—  v 

"  8 

—    V 

5  ^ 

"  g 

?  c. 

il 

Q£ 

Cfa 

Cfa 

Cfc 

Cfe 

lin   20 

273 

5.46 

297 

8.91 

335 

13.40 

390 

32.40 

432 

58.32 

480 

93.60 

lin    25 

246 

4.92 

273 

8.19 

300 

12.00 

345 

28.64 

387 

52  25 

450 

87.75 

lin    30 

220 

4.40 

249 

7.49 

270 

10.80 

312 

25.89 

351 

47.39 

390 

77.65 

lin    35 

204 

4.08 

228 

6.84 

250 

10.00 

288 

23.80 

324 

43.74 

360 

70.20 

lin    40 

192 

3.84 

216 

6.48 

237 

9.48 

272 

22.68 

306 

41.31 

330 

64.35 

lin    45 

180 

3.60 

201 

6.03 

222 

8.88 

255 

21.16 

288 

38.88 

315 

61.42 

lin    50 

174 

3.48 

192 

5.76 

210 

8.40 

243 

20.17 

272 

36.72 

300 

58.50 

lin    60 

153 

3.06 

174 

5.22 

190 

7.60 

216 

17.93 

245 

33.07 

270 

52.65 

lin    70 

144 

2.88 

162 

4.86 

177 

7.08 

204 

16.93 

229 

30.91 

252 

49.14 

lin    80 

135 

2.70 

150 

4.50 

165 

6.60 

198 

16.43 

210 

28.35 

214 

45.63 

lin    90 

129 

2.50 

144 

4.32 

156 

6.24 

180 

14.94 

201 

27.13 

222 

43.29 

1  in  100 

120 

2.40 

135 

4.05 

150 

6.00 

170 

14.11 

192 

25.92 

210 

41.16 

Diameter 

7  Inches 

8  Inches 

9  Inches 

10  Inches 

11  Inches 

12  Inches 

£~ 

•2  £• 

V 

.2  E? 

Is 

if 

i*. 

ia 

u^ 

A| 

•2  x 

•    tJ) 

o  x 

.So 

^.S 

C  j= 

5^ 

JU  *^ 

5  - 

*£  .ti 

sj 

s| 

'o  .« 

s! 

^ 

0 

0 

^ 

0 

^ 

"" 

^ 

lin   20 

510 

135.15 

540 

189 

573 

252 

620 

335 

690 

455 

750 

585 

lin   25 

480 

127.20 

480 

168 

510 

224 

540 

292 

570 

376 

600 

468 

1  in    30 

438 

116.07 

450 

158 

471 

207 

510 

275 

520 

343 

540 

420 

lin   35 

390 

103.35 

408 

143 

441 

194 

456 

246 

480 

316 

510 

397 

lin   40 

363 

96.19 

390 

137 

411 

180 

432 

233 

450 

297 

480 

374 

1  in   45 

342 

90.63 

360 

126 

390 

172 

405 

218 

430 

283 

450 

351 

1  in   50 

327 

86.65 

345 

120 

363 

160 

390 

210 

410 

270 

420 

327 

lin    60 

288 

76.32 

309 

108 

330 

145 

345 

186 

360 

238 

390 

304 

1  in    70 

270 

71.55 

280 

98 

306 

135 

324 

175 

340 

224 

360 

280 

lin    80 

252 

66.78 

270 

94 

294 

123 

309 

167 

325 

214 

330 

257 

lin    90 

240 

63.60 

258 

90 

273 

120 

285 

154 

300 

198 

315 

245 

I  in  100 

221 

58.56 

245 

86 

258 

114 

270 

146 

288 

190 

300 

234 

NOTE— To  determine  discharge  in  U.  S.  gallons  multiply  cubic  feet  by  7.5. 


72 


SEWAGE  PURIFICATION  AND  DISPOSAL 


areas,  and  are  extended  full  size  from  the  main  conduit  to 
near  the  filter  walls. 

The  drainage  system  for  a  filter  bed  is  shown  in  Fig. 
24.  In  this  system,  the  drains  are  made  of  ordinary  salt- 
glazed  sewer  pipe,  such  as  are  used  for  house  sewers,  with 

the  exception  that  the  bot- 
torn  portion  of  the  hubs  is  cut 
°^  straight  across  and  flush 
?-Mam Drain.  with  the  bottom  of  the  pipe, 
so  the  lengths  will  have  a 
firm  bearing  on  the  floor  of 
the  filter.  The  lengths  of 
pipe  forming  the  system  of 
underdrains  are  laid  with  a 
space  of  i  inch  between  their 
ends  to  provide  openings  for 
the  effluent  to  enter  the 
drains.  The  ends  of  the 
laterals  in  some  filters  are 
turned  up  and  extended  to 
above  the  water  line  in  the 
filter,  so  that  air  will  have 
free  access  at  all  times  to 
the  underdrains.  This  prac- 
tice is  not  adhered  to  in  all 
intermittent  filters,  but  in 
sprinkling  filters  and  in  con- 
tact beds  this  or  some  other 
means  of  admitting  air  to 
the  underdrains  is  advisable. 
The  branches  in  an  underdrainage  system  are  propor- 
tioned according  to  the  distance  they  are  spaced  apart  and 
the  area  they  drain.  Ordinarily,  pipes  3  inches,  4  inches,  5 
inches  and  6  inches  in  diameter  are  used,  and  they  are 
spaced  from  10  feet  to  20  feet  apart,  according  to  the  size 
of  pipe  and  the  area  they  drain.  The  grade  at  which 
underdrains  are  laid  depends  on  the  slope  given  to  the 
bottom  of  the  filter.  Generally,  the  filter  bottom  is  laid 


Fig.  24 


SEWAGE    PURIFICATION    AND    DISPOSAL 


with  a  grade  of  i  foot  in  80  to  100  feet  and  the  pipes  forming 
the  underdrainage  system  are  laid  on  the  floor  of  the  filter. 
Tile  Pipe  for  Underdrains — A  tile  pipe  such  as  is  used  for 
an  underdrainage  sys- 
tem in  an  intermittent 
filter  is  shown  in  Fig. 
25.     This  is  simply  an 
ordinary    salt-glazed 
sewer  pipe  with  part 
of  the  hub,   where   it 

rests  on  the  floor  of  the  25 

filter,  broken  away. 

Brick  Underdrains  —  In  some  filters,  in  place  of  using 
pipe  Underdrains,  drainage  is  provided  by  constructing  a 
floor  of  two  courses  of  common  bricks.  In  this  system  of 
underdrainage,  an  illustration  of  which  is  shown  in  Fig.  26, 
the  first  course  of  bricks  is  set  on  edge  and  spaced  about 
two  inches  apart,  while  the  bricks  in  the  second  course  are 
laid  flat  to  provide  a  floor  for  the  filtering  material  to  rest 
upon.  All  of  the  bricks  are  laid  with  spaces  between  to 
provide  drainage  openings  to  the  channels  below  formed 
by  the  lower  tiers  of  bricks. 

Perforated  Tile  Underdrains — A 
type  of  underdrain  tile  extensively 
used  in  the  construction  of  slow- 
sand  filters  for  water  purification, 
and  used  to  a  limited  extent  in  the 
construction  of  sewage  filters,  is 
shown  in  Fig.  27.  The  units  mak- 
ing up  this  system  of  underdrain- 
age are  laid  with  open  joints  and 
cover  the  entire  filter  bottom.  Both 
perforated  tile  underdrains  and 
brick  underdrains  are  more  expen- 
sive than  ordinary  sewer  pipe  underdrains,  and  for  inter- 
mittent filters  possess  no  advantages  that  under  ordinary 
conditions  would  justify  their  use.  In  sprinkling  filters 
however  they  prove  very  satisfactory,  as  by  their  use  a 
greater  degree  of  aeration  of  the  bed  can  be  maintained. 


Fig.  26 


74 


SEWAGE    PURIFICATION    AND    DISPOSAL 


Split=Tile  Drains — Underdrains  for  sprinkling  filters 
are  generally  made  of  split-tile,  similar  to  that  shown  in 
Fig.  28.  When  this  type  of  drain  is  used  the  tile  are  laid 

with  open  joints,  as  in  the 
/vv-.  •••."•/•.•  ..-i  case  of  tile  pipe,  but  un- 
like the  system  of  tile  pipe 
drains,  which  only  has 
branches  leading  to  a 
main  line,  when  split  tile 
drains  are  used,  the  filter 
bottom  is  completely  cov- 
ered with  the  tile  which 
are  laid  with  their  longi- 
tudinal axes  .in  the  direc- 
tion of  the  flow  of  the 
effluent.  The  effluent  fol- 
lows the  slope  of  the  floor  to  a  collector  located  either  at 
the  center  or  at  one  side  of  the  filter  basin.  The  bottom 
edges  of  split-tile  drains  are  scalloped  to  provide  spaces  for 
the  effluent  to  enter  the  drains.  The  tiles  are  bedded 
firmly  on  the  floor  of  the  filter  and  the  rows  are  placed  at 
least  one  inch  apart. 


Fig.  27 


FILTER  BEDS 

Filtering  Materials — Many  materials  have  been  success- 
fully used  in  filter  beds  for  the  filtration  of  sewage.  Among 
the  materials  most  commonly  em- 
ployed are  crushed  stone,  coke 
breeze,  coal,  cinders,  gravel  and 
sand.  The  selection  of  a  filtering 
material  depends  to  a  great  extent 
on  the  type  of  filter  to  be  con- 
structed, the  rate  of  filtration  and  degree  of  purity  desired, 
and  the  availability  of  the  different  materials  where  the 
purification  plant  is  to  be  constructed.  Sprinkling  filters 
are  generally  operated  at  a  much  higher  rate  than  are 
intermittent  filters,  consequently  a  coarse  grade  of  material 


Fig.  28 


SEWAGE    PURIFICATION    AND    DISPOSAL  75 

must  be  used  to  permit  the  rapid  flow  of  sewage  through 
the  bed  and  at  the  same  time  provide  space  for  aeration  of 
the  effluent.  The  coarser  the  filtering  material  and  the 
higher  the  rate  of  filtration,  the  less  purification  is  effected, 
consequently  when  a  great  degree  of  purity  is  desired, 
intermittent  filters  constructed  of  sand  will  be  found  the 
most  effective.  Sand  is  the  only  material  used  for  an  inter- 
mittent bed. 

The  principal  requirement  of  a  filtering  material  is 
permanency ;  any  material  that  will  soften  or  disintegrate 
with  use  is  unsuitable  for  such  purposes,  for  after  a  time 
the  disintegrated  material  will  settle  down  and  become 
compact  in  the  bottom  of  the  bed,  thus  clogging  the  filter. 


SAND 

Classification  of  Sands — Sand  is  composed  principally 
of  quartz  grains  and  hard  silicates  that  have  been  finely 
granulated  by  glacial  action,  or  by  the  attrition  of  running 
water.  Sand  can  be  obtained  from  the  sea-shore,  from 
river  beds  or  from  sand  banks.  As  it  occurs  in  its  natural 
bed,  sand  frequently  is  mixed  with  clay,  loam,  vegetable 
matter,  limestone,  or  with  other  foreign  substances,  any 
of  which,  except  limestone,  should  be  removed  before  the 
sand  is  suitable  for  filtration  purposes. 

If  sand  containing  clay  or  loam  is  used,  the  clay  or 
loam  will  cement  the  grains  of  sand  together  and  cause 
subsequent  clogging  of  the  filter.  Vegetable  matter  is 
also  objectionable  when  mixed  throughout  the  entire 
depth  of  the  filter  bed.  If  the  vegetable  matter  were  only 
in  the  top  layer  of  sand  it  could  be  reduced,  as  is  all  organic 
matter  on  the  filter.  Clay,  loam  and  vegetable  matter  can 
be  removed  from  sand  by  washing,  and  if  sand  that  is  free 
from  such  impurities  cannot  be  obtained  from  sand  bank, 
river  bed  or  seashore,  it  should  be  washed  before  being  used. 

Sand  containing  limestone  is  objectionable  as  a  filter- 
ing material  for  a  water  supply  on  account  of  imparting 


76  SEWAGE    PURIFICATION    AND     DISPOSAL 

a  certain  degree  of  hardness  to  the  water.  For  sewage 
purification,  however,  particularly  in  a  soft-water  region, 
a  moderate  degree  of  limestone  in  the  sand  instead  of 
being  harmful  is  actually  beneficial,  as  it  provides  a  base 
for  the  conversion  of  nitric  acid  into  nitrates.  The  pres- 
ence of  limestone  in  sand  can  be  determined  by  wetting 
the  sand  with  hydrochloric  acid;  if  it  gives  off  a  gas,  the 
reaction  indicates  the  presence  of  lime,  the  amount  of 
which  can  be  judged  by  the  amount  of  gas  given  off  and 
the  appearance  of  the  sample  after  the  test.  Sand  taken 
from  a  sand  bank  usually  has  sharper  and  more  angular 
grains  than  sand  which  has  been  exposed  to  the  action  of 
water.  Sea-shore  and  river-bank  sand  usually  is  composed 
of  grains  that  are  more  or  less  spherical  and  the  angles 
of  which  are  rounded  by  attrition.  All  other  conditions 
being  equal,  a  sharp  angular  sand  is  better  for  filtration 
purposes  than  is  a  smooth,  round  sand. 

Uniformity  Coefficient  of  Sand— It  is  obvious  that  all 
grains  in  a  bed  of  sand  are  not  of  equal  size,  and  this 
lack  of  uniformity  must  be  taken  into  consideration  when 
estimating  the  capacity  of  filters  and  the  quality  of  the 
effluent  likely  to  be  obtained. 

It  was  found  by  the  Massachusetts  Board  of  Health 
in  the  experiments  they  conducted  at  Lawrence,  that, 
within  certain  limits,  a  sand  in  which  the  grains  are  of 
different  sizes,  will  be  more  effective  than  will  a  sand 
with  grains  of  uniform  size.  This  increased  effectiveness 
can  be  explained  by  the  fact  that  in  a  sand  of  various 
sizes  the  sewage  must  pass  around  the  larger  grains  and 
between  the  smaller  openings  of  the  finer  particles  that 
pack  in  between  the  larger  sand  grains.  In  other  words, 
the  interstices  between  the  grains  are  smaller  when  a 
mixed  sand  is  used  than  when  the  grains  are  of  uniform 
size.  It  follows,  therefore,  that  the  finer  portion  of  the 
sand  determines  the  value  of  the  sand  for  filtration,  and 
the  nearer  to  the  right  proportion  this  fine  sand  is  to  the 
whole,  the  nearer  perfect  is  the  sand.  The  proportion  of 
coarse  to  fine  sand  is  known  as  the  uniformity  coefficient. 


SEWAGE    PURIFICATION    AND    DISPOSAL  77 

That  is,  the  uniformity  coefficient  of  a  sand  is  the  ratio 
of  the  size  of  grain  which  has  60  per  cent,  finer  than 
itself  to  the  size  that  has  10  per  cent,  finer  than  itself. 

In  intermittent  filter  beds  the  uniformity  coefficient 
should  be  as  low  as  possible,  and  if  sand  of  the  right 
quality  -cannot  be  obtained  in  the  locality,  it  may  be  worked 
over  until  the  uniformity  coefficient  is  about  right;  if  the 
uniformity  coefficient  be  too  high,  sufficient  of  the  larger 
grains  or  pebbles  can  be  removed  by  screening  to  reduce  the 
uniformity  coefficient  to  the  desired  standard.  If,  on  the  other 
hand,  the  sand  be  too  fine,  the  finer  particles  can  be  removed 
by  washing  the  sand,  thus  raising  the  uniformity  coefficient. 

Effective  Size  of  Sand — In  speaking  of  the  size  of  sand 
the  effective  size  is  referred  to.  The  effective  size  of  sand 
is  such  a  size  of  grain  that  10  per  cent,  of  the  sand  grains 
are  smaller  and  90  per  cent,  of  the  sand  grains  are  larger 
than  the  size  of  grain  that  is  known  as  the  effective  size. 
In  practice  the  effective  size  of  sand  is  found  by  sifting  a 
definite  weight  of  sand,  usually  100  grammes,  through 
a  set  of  nested  sieves,  and,  beginning  with  the  catch  on 
the  bottom  sieve,  adding  the  sand  that  passed  through  the 
bottom  sieve  and  the  catch  on  each  succeeding  sieve  until 
10  per  cent,  or  10  grammes  of  the  entire  amount  of  sand  is 
collected.  The  size  of  sieve  that  holds  back  the  90  per  cent, 
and  allows  the  10  per  cent,  to  pass  through,  determines 
the  effective  size  which  usually  is  expressed  in  millimeters. 

The  effective  size  of  sand  best  suited  to  filtration  pur- 
poses will  be  found  not  smaller  than  0.20  millimeter  in 
diameter,  nor  larger  than  0.35  millimeter  in  diameter. 
Sand  having  an  effective  size  of  0.26  millimeter  has  been 
found  very  favorable  for  the  intermittent  filtration  of 
sewage.  In  measuring  the  size  of  sand,  it  is  assumed  that 
the  grains  are  spherical  and  possess  a  diameter  that  is  equal 
to  the  cube  root  of  three  axes,  one  of  which  is  the  longest 
and  the  other  two  taken  at  right  angles  to  the  longitudinal 
axis  and  at  right  angles  to  each  other. 

Calculating  the  Effective  Size  of  Sand — The  effective 
size  of  sand,  also  the  uniformity  coefficient,  is  found  in  the 


78 


SEWAGE    PURIFICATION    AND    DISPOSAL 


following  manner:  It  will  be  assumed  that  100  grammes  of 
sand  have  been  passed  through  a  set  of  nested  sieves  and 
the  catch  of  the  various  sieves  weighed  and  found  to  be 
as  tabulated  below. 

As  the  effective  size  of  sand  is  that  of  which  10  per 
cent,  by  weight  is  smaller,  and  90  per  cent,  larger,  than 
itself,  the  effective  size  in  this  case  must  lie  between  the 
o.i  per  cent,  that  passed  the  .27  millimeter  screen  and 


Number 
of 
Sieve 

Size  of 
Screen  in 
Millimeters 

Percentage  of 
Sand  Passing 
each  Sieve 

100 

0.1G 

0.0 

80 

0.19 

o.o 

60 

0.27 

0.1 

40 

0.46 

13.5 

20 

0.88 

96.0 

16 

1.16 

99.4 

10 

2.04 

99.6 

8 

2.74 

100.0 

the  13.5  per  cent,  that  passed  the  .46  millimeter  screen; 
by  interpolation  it  will  be  found  that  the  effective  size  is 
equal  to  .41  millimeter. 

Calculating  the  Uniformity  Coefficient  of  Sand— The 
uniformity  coefficient  of  sand  is  found  by  dividing  that  size 
of  sand  of  which  60  per  cent,  by  weight  is  either  equal  to 
or  less,  by  the  effective  size  of  sand. 

In  the  present  example,  the  size  of  sand  of  which  60 
per  cent,  by  weight  is  equal  to  or  less,  lies  somewhere  be- 
tween the  13.5  percent,  that  passed  the. 46  millimeter  sieve 
and  the  .96  per  cent,  that  passed  the  .88  millimeter  sieve. 
By  interpolation  it  will  be  found  that  the  size  of  sand  is 
equal  to  .69  millimeter,  and  .69  millimeter  divided  by 
.41  millimeter,  which  is  the  effective  size  of  sand,  gives 
1.7  millimeters  which  is  the  uniformity  coefficient  of  the 
sand. 

If  all  the  grains  of  sand  in  a  filter  bed  were  of  abso- 
lutely the  same  size,  the  uniformity  coefficient  would  be  i. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


79 


With  most  sands,  however,  the  coefficient  ranges  from  2 
to  3,  while  a  uniformity  coefficient  of  from  1.6  to  2.5  will 
be  found  suitable  for  sewage  filtration. 

Nested  Sieves — A  convenient  set  of  sieves  for  screening 
sand  is  shown  in  Fig.  29.  This  set  consists  of  eight  sieves, 
the  largest  of  which  is  about  2 
inches  in  diameter,  while  the  rest 
of  the  set  are  graduated  in  size 
according  to  the  size  of  mesh,  so 
that  when  "nested"  together, 
the  smallest  sieve  having  the 
coarsest  screen  is  at  the  top  and 
the  largest  sieve  having  the 
finest  screen  is  at  the  bottom  of 
the  set.  Beginning  at  the  top, 
the  sieves  are  numbered  8,  10, 
16,  20,  40,  60,  80  and  100  respec- 
tively. 

The  number  of  a  sieve  refers  to  the  number  of  strands 
of  wire  per  lineal  inch.  For  instance,  a  No.  100  sieve  has 
100  strands  of  No.  40  Stubbs  gauge  wire  per  lineal  inch, 
and  as  100  wires  cross  at  right  angles,  there  are  100x100= 
10,000  openings  per  square  inch. 


Fig.  29 


TABLE  VIII — SIZE  AND  RATING  OF  SIEVES 


Number  of  Sieves 
also  Wires  per 
Lineal  Inch 

Openings 
per 
Square  Inch 

Size  of 
Openings  in 

Millimeters 

100 

10,000 

0.16 

80 

6,400 

0.19 

60 

3,600 

0.27 

40 

1,600 

0.46 

20 

400 

0.88 

16 

256 

1.16 

10 

100 

2.04 

8 

64 

2.74 

The  size  of  opening  in  millimeters  corresponding  to 
the  number  or  mesh  of  sieve  can  be  found  in  Table  VIII. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


Standardizing  Sieves — Sufficient  dependence  cannot  be 
placed  on  the  trade  number  of  sieves  to  warrant  their 
use  without  first  being  standardized.  To  standardize  a 
sieve,  a  definite  number  of  sand  grains,  those  that  are  the 
last  to  pass  through  the  sieve  and  which  consequently 
more  nearly  correspond  to  the  diameter  of  the  mesh  of  the 
sieve,  are  taken.  These  grains  are  counted,  and  their  sizes 
may  then  be  determined  either  by  micrometer  measure- 
ments or  by  the  weight  of  the  particles.  The  sizes  of  the 

grains  over  o.  10  millimeter  in  diam- 
eter are  more  easily  found  by 
weight,  while  the  sizes  of  grains 
smaller  than  o.  10  millimeter  in 
diameter  are  more  easily  found  by 
micrometer  measurement.  Micro- 
meter measurements  are  made  by 
taking  the  long  diameter  and  the 
two  middle  diameters  of  the  grains 
at  right  angles  to  the  long  diameter 
and  to  each  other,  as  seen  in  a 
microscope.  The  three  diameters 
are  then  measured  by  a  micro- 
meter screw,  and,  assuming  that 
the  grain  is  a  sphere  and  possesses 
Fi&-  3°  a  diameter  that  is  equal  to  the  cube 

root  of  the  three  axes,  the  mean  diameter  is  obtained  by 
taking  the  cube  root  of  the  product  of  the  three  measured 
diameters. 

EXAMPLE — What  is  the  size  of  a  sand  grain  that  measures  .30 
millimeter  along  its  long  axis,  .25  millimeter  along  its  middle  axis  and 
.20  millimeter  along  its  short  axis  ? 

SOLUTION— .30X-25X. 20=. 015,000  and  |/."oi5000— • 2466 millimeter. 
Answer. 

To  determine  the  size  of  sand  grains  by  the  weight 
method,  sift  a  certain  quantity  of  sand,  say  100  grammes, 
through  a  sieve,  weigh  the  sand  and  count  the  grains. 
The  weight  of  the  sand  divided  by  the  number  of  grains 
will  give  the  average  weight  of  each  grain.  Having  the 


SEWAGE    PURIFICATION    AND    DISPOSAL 


81 


weight  of  the  grains  the  diameter  can   be   found  by  the 
formula :  * 


d=.9 

In  which  d=diameter  of  grain  in  millimeters 
w= weight  of  grain  in  milligrams. 

EXAMPLE — What  is  the  diameter  of  a  sand  grain  that  weighs  .02017 
milligram  ? 

SOLUTION— w=. 02017  then, 

3  

d  =  .9  \/. 02017 =.242  millimeter.    Answer. 

Having  determined  the  size  of  grain  that  will  just  pass 

through  the  mesh  of  a  sieve, 
the  size  of  mesh  in  millimeters 
should  be  stamped  on  the  out- 
side of  the  sieve  which 
— j  is  then  standardized. 


Fig.  31 


Scales  for  Weighing  Sand — A  convenient  pocket  scale 
for  measuring  sand  is  shown  in  Fig.  30.  Such  an  instru- 
ment is  used  by  prospectors  and  others  outside  of  a  labora- 
tory. It  can  be  purchased  from  any  manufacturer  or 
dealer  in  chemists'  supplies,  and  may  be  had  in  a  leather 
or  wooden  case  in  a  convenient  size  to  carry  in  the  pocket 
for  field  work.  In  ordering,  assayers'  weights  should  be 
specified. 

SEWAGE  DISTRIBUTORS 

In  order  that  sewage  may  be  distributed  uniformly  to 
all  parts  of  a  filter  bed,  thus  insuring  an  equal  burden  on 
all  the  filtering  material,  systems  of  distributors  are  em- 
ployed which  are  so  proportioned  as  to  insure  an  even  dis- 
tribution of  the  sewage.  A  system  of  distributors  that  has 

*  Hazen.  In  deriving  this  formula,  the  specific  gravity  of  sand  was  taken 
as. 2.65,  that  being  the  specific  of  gravity  sands  used  in  the  Lawrence,  Mass., 
experimental  niters. 


82  SEWAGE    PURIFICATION    AND    DISPOSAL 

been  found  very  satisfactory  in  practice  for  distributing 
sewage  to  narrow  beds,  is  shown  in  Fig.  31.  These  distrib- 
utors may  be  made  either  of  cement  or  of  2 -inch  planks. 
Hinged  gates  or  sluice  gates  should  be  provided  on  dis- 
tributors so  the  flow  can  be  regulated  or  entirely  cut  off 
from  any  part  of  the  filter  bed  which  is  being  too  heavily 
dosed.  The  surface  of  the  filter  bed  when  sewage  is  dis- 
charged from  the  distributors  should  be  protected  so  the 
sand  will  not  be  disturbed  at  these  points.  Protection 
usually  is  effected  by  paving  the  surface  with  bricks,  paving 
stones,  or  by  putting  a  concrete  or  stone  flag  at  the  end 
of  each  branch  from  the  distributor.  Fig.  32  shows  in 


Fig.  32 

detail  the  construction  of  distributing  sluices.  The  bottom 
of  the  troughs  is  made  of  Portland  cement  concrete,  mixed 
in  the  proportion  of  i  cement,  3  sand  and  5  stone  or  gravel. 
The  flags  are  6  inches  thick,  8  feet  long,  and  are  laid  with 
tarred  paper  in  the  joints  between  sections  to  provide  for 
temperature  changes.  Iron  bars  bedded  in  the  concrete  are 
turned  up  at  the  edges  as  supports  for  2 -inch  plank  sides. 
Adjustable  wooden  gates  are  provided  at  the  outlets  to 
regulate  the  discharge  if  it  is  desired  to  throw  unequal 
quantities  on  different  parts  of  the  bed.  A  2 -inch  plank 
stop  in  front  of  ea'ch  opening  checks  the  velocity  of  the 
influent  so  it  will  not  wash  the  surface  of  the  filter. 

A  distributor  system  designed  to  distribute  sewage  to 
all  parts  of  a  filter  bed  is  shown  in  Fig.  33. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


83 


In  small  or  narrow  filter  beds,  the  distributor  shown 
in  a  previous  illustration  will  be  found  satisfactory,  but 
in  filter  beds  of  large  area  branch  distributors  are  desirable 
to  prevent  over  dosing  of  part  of  the  filter,  while  the  rest 
of  the  bed  remains  undosed.  It  will  be  noticed  that  the 
main  sluice  is  decreased  in  size  as  branches  are  taken 
off,  so  as  nearly  as  possible  to 
distribute  the  fluid  equally  over 
the  entire  filter  surface. 

A  section  of  a  cheaper 
type  of  distributing  sluice  is 
shown  in  Fig.  34.  This  dis- 
tributor is  made  entirely  of 
wood,  and  discharges  the 
sewage  onto  the  beds  through 
scuppers  in  the  sides  of  the 
sluices.  Passing  beneath  the 
distributors  at  certain  inter- 
vals are  wooden  cross-pieces, 
to  which  the  bottom  of  the  dis- 
tributor is  securely  fastened  to 
prevent  warping.  The  cross- 
pieces  extend  beyond  the  sides 
of  the  distributors  a  sufficient 
distance  to  catch  the  flow  of 
sewage  from  the  scuppers  and 
thus  prevent  surface  washing 
of  the  sand  bed.  Shear  gates 
are  provided  at  all  branch 
sluices  to  divert  the  flow  of 
sewage  from  the  main  dis- 
tributor into  the  branches.  Wood  is  not  an  ideal  material  for 
distributing  sluices.  If  a  soft  wood  suitable  in  other  ways 
for  this  purpose  is  used,  the  distributors  are  liable  to  be- 
come saturated  with  sewage  and  give  rise  to  objectionable 
odors.  The  element  of  cheapness,  however,  will  recom- 
mend this  type  of  distributor,  and  when  the  purification 
plant  is  removed  a  sufficient  distance  from  highways  or 


Fig.  33 


84  SEWAGE    PURIFICATION    AND    DISPOSAL 

habitations  the  additional  odor  from  saturated  sluices  will 
hardly  be  noticeable. 

Proportioning  Distributors — In  designing  distributors 
for  intermittent  filters,  they  should  be  so  proportioned  that 
each  branch  will  receive  its  share  of  the  total  flow  and  the 
main  distributor,  together  with  the  branches,  should  be  of 
sufficient  size  to  discharge  within  a  comparatively  short 
time  the  entire  contents  of  a  dosing  chamber.  If  the 
sewage  is  discharged  onto  the  filter  bed  in  a  small  stream, 
the  liquid  portion  will  sink  into  the  sand  within  a  short 
radius  of  the  point  of  discharge,  thereby  overdosing  that 
portion,  while  the  rest  of  the  area  remains  unused.  When 
discharged  in  a  large  quantity  within  a  few  minutes'  time, 
the  sewage  floods  the  entire  filter  area,  thereby  imposing 
on  each  part  its  proportionate  share  of  the  purification 
to  be  affected. 


Fig.  34 


The  size  of  the  distributing  main  for  an  intermittent 
filter  depends  on  the  quantity  of  sewage  that  is  to  be  dis- 
charged in  a  given  time.  When  a  filter  treats  the  effluent 
from  a  septic  tank,  the  quantity  of  sewage  to  be  conducted 
per  minute  will  depend  on  the  discharging  capacity  of  the 
automatic  siphon.  Ordinarily,  in  large  plants  the  siphon 
is  proportioned  to  empty  the  dosing  chamber  in  about  ten 
minutes ;  so  by  dividing  the  capacity  of  the  dosing  chamber 
in  cubic  feet  by  ten  minutes  will  give  the  quantity  of  sewage 
in  cubic  feet  to  be  removed  per  minute. 

Having  the  quantity  to  be  removed  per  minute,  a 
siphon  having  the  required  capacity  can  be  found  by  the 
formula 


SEWAGE    PURIFICATION    AND    DISPOSAL  85 


d=.234|/q      l 

In  which  d=diameter  of  siphon  in  feet 

q=cubic  feet  to  be  discharged  per  second 
l=length  of  siphon  in  feet 
h=head  in  feet 

In  this  formula,  the  head  h  is  taken  as  the  difference 
in  length  between  the  long  and  short  legs  of  the  siphon. 
As  a  matter  of  fact,  when  the  siphon  is  first  brought  into 
action,  water  flows  through,  due  to  the  hydraulic  head, 
and  is  not  siphoned  out,  the  siphon  being  brought  into 
requisition  only  when  sewage  is  lowered  to  the  top  of  the 
overflow  pipe  within  the  bell. 

EXAMPLE — What  diameter  of  siphon  will  be  required  to  discharge 
8  cubic  feet  of  sewage  per  second  through  a  siphon  12  feet  long,  acting 
under  a  head  of  three  feet  ? 

SOLUTION — Substituting  the  values  given  in  the  example  in  the 
formula 

5     

d=.234  |/64^  12=-234  X  3.031  =  1  foot  diameter.     Ans. 

Siphons  are  made  in  stock  sizes  that  coincide  with  the 
standard  sizes  of  pipe,  and  if  the  calculated  size  of  a  siphon 
is  between  two  stock  sizes,  the  larger  size  should  be  used. 

The  velocity  of  flow  through  an  automatic  siphon,  such 
as  is  used  for  the  discharge  of  effluent  from  a  dosing 
chamber,  is  about  10  feet  per  second  at  the  beginning  of  the 
operation  and  about  8  feet  per  second  at  the  end  of  siphonic 
action ;  thus  a  large  quantity  of  sewage  at  a  comparatively 
high  velocity  is  discharged  in  a  short  period  of  time  on  the 
distributors,  which  are  laid  at  such  a  grade  that  the  velocity 
of  the  flow  in  them  seldom  exceeds  4  feet  per  second  and 
will  perhaps  average  3  feet  per  second.  This  decreased 
velocity  should  be  taken  into  consideration  when  propor- 
tioning the  distributor  system,  and  the  flumes  made  large 
enough  to  care  for  the  sewage  when  flowing  at  a  velocity 
of  only  3  feet  per  second.  When  the  quantity  of  sewage 
and  the  velocity  of  flow  are  known,  the  cross-sectional  area 


86  SEWAGE    PURIFICATION    AND    DISPOSAL 

of  a  rectangular  flume  to  care  for  that  quantity  of  sewage 
can  readily  be  calculated  by  the  formula 

a=q-:-v 
In  which  a = cross-sectional  area  of  distributor  in  square  feet 

q=quantity  of  sewage  in  cubic  feet  per  minute 

v= velocity  in  feet  per  minute 

A  flume  proportioned  to  care  for  the  sewage  discharged 
by  the  siphon  in  the  preceding  example  and  solution,  is  cal- 
culated in  the  following  example  and  solution : 

EXAMPLE — What  should  be  the  cross-sectional  area  of  a  rectangular 
distributing  flume  to  discharge  8  cubic  feet  of  sewage  per  second  at  a 
velocity  of  3  feet  per  second  ? 

SOLUTION — Quantity  of  sewage  to  be  cared  for  per  minute=8x60= 
480  cubic  feet.  Velocity  of  flow  per  minute =3x60 =180.  Substituting 
those  quantities  in  the  formula 

a =480  -^180 =2. 7  square  feet. 

2.7  square  feet  would  be  equal  to  a  flume  4  feet  wide 
by  8  inches  in  depth,  which  would  be  about  the  right  propor- 
tion for  a  distributing  flume.  To  provide  against  an  over- 
flow of  effluent  from  the  distributor  should  it  be  lined  with 
ice  or  otherwise  obstructed,  the  sides  are  usually  made  10 
inches  high  to  allow  a  suitable  margin  of  safety. 

Size  of  Half-round  Flumes — The  size  of  a  half-round 
flume  required  to  conduct  a  given  quantity  of  sewage  can 
be  calculated  by  the  formula  for  finding  the  size  of  under- 
drain  to  discharge  a  given  quantity  of  effluent  when  running 
half  full.  It  is  obvious  that  a  pipe  which  will  conduct  100 
cubic  feet  of  liquid  per  minute  when  running  half  full,  will 
be  equal  in  diameter  to  a  half-round  flume  which  will  con- 
duct the  same  amount  when  running  full.  Knowing  the 
size  of  flume  that  will  conduct  a  certain  quantity  of  fluid 
when  running  full,  using  the  next  larger  commercial  size 
will  allow  a  margin  of  safety  that  will  be  ample  and  will 
not  overflow. 

The  size  of  pipe  required  to  conduct  a  certain  quantity 
of  liquid,  when  laid  at  different  grades  and  only  half  full, 
can  be  found  by  doubling  the  quantity  of  sewage  to  be  con- 
ducted, and  finding  in  Table  VII  the  size  of  pipe  that  will 
care  for  that  quantity. 


SEWAGE    PURIFICATION    AND    DISPOSAL  87 

AERATORS 

Object  of  Aerators — When  sewage  effluent  passes  from 
a  septic  tank,  the  fluid  is  devoid  of  oxygen.  Up  to  this 
point,  the  process  of  decomposition  has  been  an  anaerobic 
one  in  which  the  nitrogeneous  solids  have  been  broken 
down  into  ammonic  compounds,  while  the  carbonaceous 
solids  have  been  converted  into  carbon  dioxide,  alcohol, 
water,  succinic  acid  and  glycerine.  The  further  process 
of  sewage  purification  is  an  oxidizing  one  in  which  anaero- 
bic bacteria  are  the  oxidizing  mediums  which  convert  the 
ammonia  and  other  compounds  into  useful  nitrates.  This 
part  of  the  process  is  known  as  nitrification,  and  in  order 
that  the  most  favorable  conditions  be  provided  for  the 
nitrifying  organisms,  the  sewage  should  be  saturated  with 
air  before  being  discharged  onto  the  filter  beds.  Tests  of 
sewage  for  oxygen  show  that  before  entering  a  septic  tank 
the  sewage  contains  from  o  to  5  per  cent,  of  its  capacity  for 
air;  upon  leaving  the  septic  tank  the  effluent  is  utterly 
devoid  of  air;  after  aeration  the  effluent  contains  about  75 
per  cent,  of  its  capacity  for  air,  and  when  applied  to  the 
filter  beds  about  40  per  cent,  of  its  capacity  for  air. 

Aeration  is  not  always  provided  for.  When  there  is 
sufficient  fall  between  the  septic  tank  and  the  filter  bed 
aeration  is  easily  accomplished  by  discharging  the  effluent 
over  a  weir  or  by  passing  the  effluent  through  an  aerator, 
but  when  the  fall  is  very  slight,  the  only  aeration  that  can 
be  effected  is  that  obtained  by  placing  deflectors  in  the 
distributors  to  deflect  the  effluent  from  side  to  side,  thus 
exposing  as  far  as  possible  all  particles  to  the  atmosphere. 
Aerators  are  sometimes  objectionable  on  account  of  the 
objectionable  odors  they  release  in  the  form  of  gas. 

Example  of  an  Aerator — An  aerator  that  has  been  suc- 
cessfully used  in  practice  is  shown  in  Fig.  35.  This  appa- 
ratus consists  simply  of  a  standpipe,  to  which  is  attached 
a  series  of  perforated  baffle  plates  that  break  up  the  stream 
of  liquid  into  thin  films,  small  streams  and  drops,  thus 
exposing  all  parts  to  the  atmosphere.  After  flowing  over 


88 


SEWAGE    PURIFICATION    AND    DISPOSAL 


or  dripping  through  the  plates,  the  liquid  settles  to  the  bot- 
tom of  the  aerator  chamber  and  flows  through  an  outlet  into 
the  collecting  gallery.  From  here  the  liquid  can  be  dis- 
charged through  one  of  the  sluice  gates  into  the  dosing 
chamber,  or  bi-passed  around  the  dosing  chamber  either  to 
the  filter  beds  or  to  the  sewer  outfall.  By  opening  the 
sluice  gate  which  closes  the  effluent  pipe,  the  septic  liquid 
can  be  passed  through  to  the  dosing  chamber  or  filter 
beds  without  passing  through  the  aerator.  A  high  degree 
of  aeration  can  be  effected  by  causing  the  liquid  to  overflow 


Fig.  85 


a  standpipe  similar  to  the  one  shown  in  the  illustration,  but 
without  the  perforated  baffle  plates  to  break  up  the  stream. 
Any  form  of  sluice  which  will  break  up  a  column  of  water 
so  as  to  expose  it  to  the  atmosphere  in  thin  films,  drops  or 
spray,  will  prove  an  effective  aerator. 

Size  and  Capacity  of  Intermittent  Filters — Filter  beds 
tisually  are  proportioned  to  the  size  of  the  dosing  chamber 
in  the  purification  plant,  or  the  dosing  chamber  and  filter 
beds  are  proportioned  to  each  other,  so  that  when  the  en- 
tire dose  is  discharged  onto  a  filter  the  sewage  will  cover 
the  surface  of  the  bed  to  a  uniform  depth.  Dosing  tanks, 
however,  are  never  made  so  large  that  they  will  require 
the  construction  of  filters  of  greater  area  than  one  acre. 


SEWAGE    PURIFICATION    AND    DISPOSAL  89 

When  the  amount  of  sewage  is  large,  it  is  found  to  be  a 
better  practice  to  decrease  the  size  of  the  dosing  chamber 
and  filter  areas  and  provide  more  beds.  Reserve  filtration 
capacity  should  always  be  provided  so  that  the  surface  of 
one  bed  can  be  cleaned  or  raked  while  the  others  are  in 
service.  For  this  reason,  a  filtration  plant  of  less  than  two 
beds  is  seldom  constructed,  even  though  one  filter  bed  has 
sufficient  area  to  care  for  the  maximum  flow  of  sewage. 

The  capacity  of  filter  beds  depends  greatly  on  the 
strength  and  staleness  of  the  sewage  and  on  the  effective 
size  of  the  sand.  Intermittent  filters  can  be  and  in  many 
localities  are  used  to  treat  crude  sewage,  in  which  respect 
they  are  quite  effective.  The  amount  of  crude  sewage, 
however,  that  can  be  purified  per  acre  of  surface  is  com- 
paratively small  and  more  economic  results  are  obtained 
where  large  quantities  of  sewage  are  to  be  purified  by 
using  intermittent  filtration  as  a  final  process  to  follow  some 
preliminary  treatment. 

Sewage  is  usually  applied  to  the  filter  beds  twice,  three 
times  or  four  times  in  twenty-four  hours,  although  with 
very  fine  sand,  sometimes  only  three  doses  a  week  is  all 
that  can  be  applied. 

When  treating  crude  sewage,  the  applications  are  made 
at  twelve-hour  intervals,  while  with  septic  sewage  the 
period  is  cut  down  to  from  six  to  eight-hour  intervals.  With 
crude  sewage,  the  dose  is  proportioned  to  cover  the  surface 
from  i  inch  to  2  inches  in  depth,  which  being  applied  every 
twelve  hours  would  be  equal  to  a  rainfall  of  from  two  inches 
to  four  inches  over  the  entire  surface  every  twenty-four 
hours.  Crude  sewage  treated  at  this  rate  would  be  equal 
to  the  purification  of  from  50,000  gallons  to  100,000  gallons 
per  acre  per  twenty-four  hours. 

When  treating  septic  sewage  or  very  weak  crude  sewage, 
the  fluid  is  applied  in  doses  of  from  2  inches  to  4  inches  in 
depth  over  the  entire  filter  surface.  It  has  been  proposed 
in  some  instances,  where  sewage  has  been  subjected  to 
septic  action  and  afterward  passed  at  a  rapid  rate  through 
a  coarse  sprinkling  filter,  to  still  further  treat  the  effluent 


90  SEWAGE    PURIFICATION    AND    DISPOSAL 

from  the  sprinkling  filter  by  passing  it  at  the  rapid  rate 
of  28  inches  per  day  through  intermittent  filters.  At  this 
rate  of  purification,  each  acre  of  filter  surface  would  purify 
750,000  gallons  of  sewage  per  acre  per  twenty-four  hours. 
The  conditions  are  unusual,  however,  and  if  sewage  be 
treated  at  that  rate,  provision  no  doubt  would  be  required 
to  scrape  the  surface  of  the  sand,  as  in  water  nitration,  and 
wash  it  for  future  use.  Where  land  is  expensive  and  the 
expense  of  labor  offsets  the  interest  that  would  have  to  be 
paid  on  land  purchased  for  additional  beds,  or  where  suffi- 
cient area  cannot  be  obtained,  this  practice  might  be  ad- 
visable. It  is  doubtful,  however,  if  sewage  will  often  have 
to  be  purified  to  such  a  degree  as  to  require  intermittent 
filtration  of  the  effluent  from  sprinkling  filters. 

For  the  treatment  of  septic  sewage  by  intermittent 
filtration  under  ordinary  conditions,  a  rate  of  1 2  inches  per 
twenty-four  hours  applied  in  three  or  four  separate  doses  is 
about  the  maximum  that  can  satisfactorily  be  purified.  A 
rate  of  1 2  vertical  inches  per  twenty-four  hours  is  equal  to 
the  purification  of  326,700  gallons  per  day  per  acre  of  sur- 
face. At  this  rate  of  purification,  and  assuming  a  per  capita 
consumption  of  100  gallons  of  water  per  day,  one  acre 
of  filter  surface  would  be  required  for  each  3,267  inhabi- 
tants. That  is  the  greatest  number  of  people,  however, 
that  one  acre  of  filtration  surface  will  care  for,  and  when 
it  is  considered  that  sand  which  is  suitable  for  that  rate  of 
filtration  is  not  always  obtainable;  that  reserve  surface 
must  always  be  provided  so  beds  that  have  been  overdosed 
can  rest;  that  communities  sometimes  have  their  popula- 
tion temporarily  increased  ten  to  twenty  per  cent,  by  the 
influx  of  visitors ;  that  ground  water  infilters  into  the  sewers, 
and  that  provision  should  be  made  for  growth  in  population, 
when  all  these  conditions  are  considered,  it  is  found  that 
provision  of  one  acre  filter  surface  for  each  2,000  inhabi- 
tants is  about  the  greatest  proportion. 

Intermittent  filtration  of  septic  sewage  at  the  rate  of 
500,000  gallons  per  acre  per  day  has  been  successfully  main- 
tained under  favorable  conditions,  and  the  tendency  of  the 


SEWAGE    PURIFICATION    AND    DISPOSAL  91 

times  is  to  increase  the  rate  of  filtration.  However,  in  the 
present  state  of  sewage  purification  practice,  lower  rates  of 
filtration  are  safer  and  are  sure  to  give  more  satisfactory 
results.  To  summarize  the  results  obtained  by  intermit- 
tent filtration,  it  might  be  stated,  that  wThen  treating  crude 
sewage  with  clear  fine  sand,  30,000  gallons  per  acre  per 
day  can  be  purified  to  such  a  degree  that  the  effluent  is 
originally  far  superior  to  ordinary  pure  water,  and  the  num- 
ber of  bacteria  per  unit  volume  is  much  less  than  in  the  pure 
water.  When  treating  septic  sewage  on  a  similar  bed,  the 
same  degree  of  purity  can  be  obtained  when  operated  at  a 
rate  of  60,000  gallons  per  acre  per  day. 

With  intermittent  filters  of  clean,  sharp,  coarse  sand 
treating  crude  sewage,  60,000  gallons  per  acre  per  day  can 
be  purified  to  such  a  degree  that  97  to  99  per  cent,  of  the 
organic  matter  and  99.9  per  cent,  of  the  bacteria  will  be 
removed,  and  the  effluent  will  be  colorless,  generally  clear 
and  will  possess  little  or  no  sediment.  On  similar  filter 
beds  120,000  gallons  of  septic  sewage  per  acre  per  day  can 
be  purified  to  an  equal  degree.  On  intermittent  filters  of 
coarse  sand  similar  to  the  beds  just  described,  180,000  gal- 
lons of  crude  sewage  or  360,000  gallons  of  septic  sewage  can 
be  filtered  per  acre  per  day,  removing  97  per  cent,  of  the 
organic  matter,  95  per  cent,  of  the  bacteria,  and  producing 
an  effluent  that  will  more  than  satisfy  the  conditions  of  any 
standard  of  sewage  purification  yet  laid  down.  Such  filters 
may  occasionally  require  a  period  of  rest  or  a  working  over 
of  the  top  layers  of  the  filtering  material,  while  in  some 
cases  it  might  be  necessary  to  remove  and  wash  the  top 
layer  of  sand. 

Where  crude  sewage  is  treated  by  intermittent  filtra- 
tion, screens  should  be  provided  to  hold  back  coarse,  in- 
soluble particles  that  would  clog  the  surface  of  the  filter 
bed. 

A  fair  idea  of  the  relation  of  size  and  uniformity  coeffi- 
cient of  sand  to  size,  frequency  and  size  of  dose  of  crude 
sewage  that  a  bed  will  purify  can  be  found  in  Table  IX, 
which  gives  the  actual  quantities  of  crude  sewage  found  the 


93 


SEWAGE    PURIFICATION    AND    DISPOSAL 


best  to  apply  to  beds  of  different  sizes  of  material.  The 
table  will.be  found  helpful  not  only  in  showing  the  actual 
amount  of  sewage  that  different  sizes  of  sand  will  purify, 
but  also  in  pointing  out  the  best  quantity  to  apply  at  a 
given  time,  the  length  of  time  which  the  bed  should  be 
allowed  to  rest  and  the  size  and  frequency  of  doses. 

The  mechanical  composition  of  the  materials  used  in 
the  various  beds,  the  results  of  which  are  tabulated  in 
Table  IX  can  be  found  in  Table  X. 

These  materials   are   supposed  to  include   the   whole 

range  of  sands  available  for  sewage  purification. 

« 
TABLE  IX — QUANTITY  OF  SEWAGE  PURIFIED  BY  DIFFERENT  SIZES  OF  SAND 

(Massachusetts  State  Board  of  Health) 


Size  of  Dose 

No.  of 
Filter 

Uni- 
formity 
of 
Coeffi- 
cient 

Effective 
Size  of 
Sand  in 
Milli- 
meters 

Depth 
of  Filter 
Bed  in 
Feet 

Number 
of  Doses 
in  One 
.  Week 

Average 
Amount 
Applied 
Dailv  in 
Galfons 
per  Acre 

Gallons 
per  Acre 

Per 
Cent,  of 
Volume 
of  Filter 

1 

1.8 

5.00 

5 

2,800 

.17 

500 

200,000 

2 

2.4 

.48 

5 

40,000 

2.45 

18 

103,000 

3 

7.8 

.35 

4 

70,000 

4.37 

6 

60,000 

4 

2.0 

.17 

5 

120,000 

7.36 

6 

103,000 

5 

2.3 

.06 

5 

140,000 

8.60 

3 

60,000 

6 

2.3 

.03 

5 

80,000 

4.91 

3 

34,000 

7 

9 

.02 

5 

The  data  contained  in  Table  X  are  plotted  in  diagram 
on  next  page,  Fig.  36.  "The  lines  representing  the  diam- 
eters are  spaced  according  to  the  logarithms  of  the  diameters 
of  the  particles,  as  in  this  way  materials  of  corresponding 
uniformity  in  the  range  of  sizes  of  their  particles  give 
equally  steep  curves,  regardless  of  the  absolute  sizes  of  the 
particles,  thus  greatly  facilitating  a  comparison  of  different 
materials.  This  scale  also  shows  adequately  every  grade 
of  material  from  o.oi  to  above  10  millimeters,  in  a  small 
space  and  without  unduly  extending  any  portion  of  the  scale. " 

The  height  of  curve  at  any  point  shows  the  per  cent,  of 
material  finer  than  the  size  indicated  at  the  bottom  of  the 
diagram. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


93 


(Mass.  State  Board  of  Health) 


It  will  be  seen  by  the  diagram  that  materials  2,  4,  5  and 
6  have  approximately  steep  curves,  and  by  referring  to 
the  table  it  will  be 
found  that,  while 
their  effective  sizes 
differ  considerably, 
their  uniformity  co- 
efficient is  approxi- 
mately the  same. 
Likewise  the  curves 
of  Nos.  3  and  7  are 
quite  similar,  and 
reference  to  the  table 
shows  that  they  have 
approximately  the 
same  uniformity  co- 

•  OI      -O3  .06 -12  .24  .46  .98    2.2      6.2    12  £ 

efficient.  Fig  36_Diameter  of  Sand  in  Millimeters 


AIR  AND  WATER  CAPACITY  OF  SANDS 

Sharp  grained  sands  having  uniformity  coefficient  of 
less  than  2,  as  ordinarily  packed,  have  nearly  45  per  cent, 
of  open  space  or  voids ;  and  sands  with  coefficients  between 

TABLE  X — MECHANICAL  COMPOSITION  OF  SANDS  AVAILABLE 

FOR  FILTRATION 
(Report  of  Massachusetts  State  Board  of  Health) 


Diameter  in  Millimeters 

%  Per  Cent. 

No.  7 

No.  G 

No.  5 

No.  4 

No.  3 

No.  2 

No.  1 

Finer  than  12.6      ... 
Finer  than     6.2      ... 
Finer  than     2.2      ... 
Finer  than       .98    .     .     . 
Finer  than       .46    .     .     . 
Finer  than      .24    ... 
Finer  than       .12    .     .     . 
Finer  than       .06    .     .     . 
Finer  than      .03    .     .     . 
Finer  than       .01  organic 
Effective  size     .... 
Uniformity  coefficient    . 

99 
96 
92 
89 
80 
67 
51 
33 
16 
6 
.02 
9 

83 
73 
57 
32 

13 
7 
4 
2 
.5 

100 
97 
85 
53 
7 
1.5 

98 

27 

100 

91 
26 
3 

ibo' 

85 
35 
10 

.03 

2.3 

100 
90 
43 
10 
2 

.06 
2.3 

.17 
2 

.35 

7.8 

.48 
2.4 

5 

1.8 

94 


SEWAGE    PURIFICATION    AND    DISPOSAL 


(Mass  State  Board  of  Health) 


36 


\ 


3  and  2  usually  have  about  40  per  cent,  of  open  space.  The 
voids  are  smaller  in  more  mixed  materials,  and  sands  with 
uniformity  coefficient  of  from  6  to  8  have  only  about  30  per 
cent,  of  open  space,  while  in  materials  with  extremely  high 
coefficients  almost  no  space  is  left.  With  water-worn, 
round-grained  sands,  the  open  space  is  from  2  to  5  per  cent, 
less  than  for  corresponding  sharp  grained  sands. 

It  is  evident  that 
the  quantity  of  water 
within  a  sand  bed 
can  never  exceed  the 
volume  of  the  voids, 
which  is  never  over 
45  per  cent.,  and  the 
water  capacity  is 
usually  considerably 
less  than  the  volume 
of  voids.  By  the 
water  capacity  of  a 
sand  is  meant  the 
amount  of  water  re- 
tained in  the  inter- 
stices after  the  bed 
has  been  thoroughly 
drained.  The  per- 
centage of  water  re- 
maining in  beds  of 
the  materials  described  in  the  foregoing  tables,  can  be  seen 
by  reference  to  the  diagram,  Fig.  37.  This  diagram  gives 
not  only  the  percentage,  by  volumes,  of  water  retained  at 
each  6  inches  depth  of  the  filter  bed,  but  also  the  air 
capacity  of  the  filter  beds.  The  full  lines  represent  water 
capacity  and  the  dotted  lines  air  capacities.  It  will  be 
noticed  that  the  curve  of  No.  6,  which  is  the  finest  material 
listed,  shows  that  the  lower  18  inches  of  the  bed  are  practi- 
cally filled  with  water  after  the  bed  has  been  thoroughly 
drained,  and  that  in  No.  i,  which  is  a  very  coarse  material, 
the  water  drains  out  almost  uniformly  at  all  depths. 


5          IO         15        2O        25       3O       35 

Fig.  87— Per  cent,  by  volume 


40      45 


SEWAGE    PURIFICATION    AND    DISPOSAL 


95 


A  classification  of  sands  which  gives  the  approximate 
size  of  sand  for  different  classifications  can  be  found  in 
Table  XI.  This  table  gives  the  size  of  some  particles  which 
are  much  smaller  than  can  be  used  for  a  filter  bed.  Sand 
having  an  effective  size  smaller  than  .03  millimeter  in  diam- 
eter is  unfit  for  filtration  purposes,  while  particles  smaller 
than  .01  millimeter  are  considered  organic  matter. 

TABLE  XI — CLASSIFICATION  OF  SANDS 


Name 

Diameter 
Millimeters 

Inches 
Approximate 

Fine  gravel  .  
Coarse  sand. 

2.0  to  1.0 
1.0  to  .5 

.08  to  .04 
.04  to   02 

Medium  sand 

.5  to  .25 

.02  to  .01 

Fine  sand  .  
Very  fine  sand  or  dust  .  .  . 
Silt  .  . 

.25  to  .1 
.1  to  .05 
.05  to  .01 

.001  to  .004 
.004  to  .002 
.002  to  .004 

Fine  silt  
Clay  .  .  .  

.001  to  .005 
.005  to  .0001 

.0004  to  .0002 
.0002  and  under 

Effect  of  Temperature  on  Intermittent  Filtration- 
Temperature  has  a  marked  influence  on  the  operation  of 
an  intermittent  filter.  As  soon  as  frost  begins  to  form  on 
a  filter  bed  a  change  can  be  detected  in  the  chemical  compo- 
sition of  the  effluent;  the  free  ammonia  increases,  the 
nitrates  decrease  and  the  organic  matter,  as  shown  by  the 
albuminoid  ammonia  and  by  the  oxygen  consumed  tests, 
also  increases  but  not  in  the  same  proportion  as  the  free 
ammonia.  During  extremely  cold  weather  nitrification 
almost  ceases  and  ammonia  instead  of  nitrates  is  largely 
the  end  product  of  oxidation  so  far  as  nitrogen  is  concerned. 

A  low  temperature  does  not  affect  to  a  great  extent  the 
first  stage  of  purification,  that  which  breaks  down  the 
organic  matter  into  ammonia  and  carbonic  acid,  conse- 
quently while  the  purification  effected  is  not  chemically 
complete,  almost  as  much  of  the  organic  matter  is  reduced 
as  during  the  warmer  months. 

Frost  has  not  so  retardant  an  effect  on  the  passage  of 
sewage  through  a  filter  as  would  be  expected ;  when  sand 


96 


SEWAGE    PURIFICATION    AND    DISPOSAL 


is  frozen,  after  draining,  there  still  remains  a  certain  per- 
centage of  open  pores,  and  when  sewage  at  a  temperature 
of  from  44  to  46  degrees  Fahr. ,  which  is  the  average  tem- 
perature of  sewage  in  winter,  is  applied  to  the  bed,  the 
sewage  easily  finds  its  way  through  the  open  pores,  thawing 
the  frost  as  it  proceeds.  After  the  sewage  has  passed  away 
there  still  remains  in  the  bed  a  certain  amount  of  water 
which  again  freezes,  but  is  again  thawed  when  the  bed 
receives  another  application  of  sewage.  Conditions  might 
be  such  at  times  that  temporarily  a  filter  bed  is  made 
inoperative  by  ice.  A  filter  bed  of  fine  sand  or  one  that  is 


Fig.  38 


surface  clogged  to  such  an  extent  that  sewage  is  retained 
on  the  surface  for  a  considerable  time  in  extremely  cold 
weather,  might  have  the  sewage  frozen  on  the  bed,  thus 
closing  all  pores.  This  condition  however  is  rare  in  prac- 
tice. Ordinarily,  filter  beds  that  are  covered  with  snow 
and  that  are  frozen  to  a  depth  of  36  inches,  cause  no 
trouble  when  sewage  is  applied,  the  effluent  appearing  in 
the  drains  within  several  hours  after  the  application.  As 
a  matter  of  fact,  snow  seems  to  form  a  blanket  over  the 
surface  of  the  filter,  and  when  sewage  is.  applied  it  flows 
under  the  snow  which  prevents  evaporation  and  consequent 
chilling  of  the  sewage.  A  further  protection  sometimes 


SEWAGE    PURIFICATION    AND    DISPOSAL  97 

is  provided  for  intermittent  filters  in  cold  climates  by 
arranging  the  surface  of  the  filter  in  furrows  as  shown  in 
Fig.  38.  Any  ice  that  forms  then  bridges  the  furrows 
from  ridge  to  ridge,  allowing  the  sewage  to  flow  beneath 
and  percolate  through  to  the  underdrains.  On  the  whole, 
it  may  be  said  that  filters  can  safely  be  operated  in  cli- 
mates where  the  mean  temperature  of  the  atmosphere  in 
winter  is  not  lower  than  18  or  20  degrees  Fahr. 

Care  of  Filter  Beds — A  certain  amount  of  material  ac- 
cumulates on  the  surface  of  filter  beds,  particularly  when 
treating  crude  sewage.  This  material  cannot  properly  be 
termed  sludge,  but  is  more  of  the  nature  of  a  dry  cake  of  a 
stable  and  inoffensive  character,  which  if  not  removed  will 
clog  the  filter  surface,  thus  reducing  greatly  the  capacity 
of  the  filter.  To  remove  this  material  the  beds  must  be 
frequently  raked  and  harrowed  and  occasionally  plowed, 
spaded  and  scraped.  It  may  be  assumed  as  a  fair  average, 
that  8  cubic  yards  of  sand  and  clogging  material  will  have 
to  be  removed  from  a  filter  surface  for  each  million  gal- 
lons of  crude  sewage  treated.  After  plowing  or  scraping 
a  filter  it  should  be  carefully  leveled,  so  that  sewage  will 
flow  to  all  parts  of  the  field. 


SPRINKLING  FILTERS 

Principles  of  Sprinkling  Filtration — The  terms  sprink- 
ling filter  and  percolating  filter  are  now  indiscriminately 
used  to  designate  a  certain  type  of  filter  plant.  In  this 
work,  the  term  sprinkling  filter  will  be  used  as  the  better 
term  to  describe  the  type.  In  intermittent  filtration,  the 
name  is  derived  from  the  method  of  applying  sewage  to 
the  filter;  by  following  the  same  system  of  nomenclature, 
the  term  sprinkling  filter  seems  the  better  one  to  designate 
filters  of  coarse  grained  materials  onto  which  sewage  is 
sprayed  through  perforated  pipes  or  sprinkler  nozzles.  The 
term  seems  the  more  suitable  one  when  it  is  considered 
that,  in  any  type  of  filter,  the  fluid  percolates  through  the 
bed,  and  all  might  equally  be  classed  as  percolating  filters. 


98  SEWAGE    PURIFICATION    AND    DISPOSAL 

Sprinkling  nitration  is  a  method  of  sewage  purifica- 
tion in  which  either  crude  sewage  or  septic  effluent  may 
be  applied  in  a  fine  spray,  like  drops  of  rain,  to  a  specially 
prepared  filter  bed  of  coarse  material.  The  material 
used  in  the  bed  of  a  sprinkling  filter  is  too  coarse  to  act 
as  a  strainer,  consequently  what  purification  takes  place 
during  the  few  minutes  consumed  by  the  sewage  in  pass- 
ing through  the  filter  bed,  is  due  to  bacterial  action  and 
the  process  is  more  of  a  biological  than  a  mechanical  one. 
Notwithstanding  the  fact  that  the  process  is  mostly  biologi- 
cal, and  that  the  material  of  the  filter  bed  is  too  coarse  to 
act  as  a  strainer,  there  is  a  certain  mechanical  action  within 
a  sprinkling  filter  which  plays  an  important  part  in  holding 
back  the  organic  matter  until  bacterial  activity  has  reduced 
it  to  a  stable  condition.  When  closely  examined,  the 
material  of  which  the  filter  bed  is  composed,  is  found  to  be 
enveloped  in  a  gelatinous  film.  This  film  is  of  such  com- 
position that  it  entangles  and  holds  back  anything  brought 
in  contact  with  it,  which  is  not  of  sufficient  weight  and 
hardness  to  cut  or  tear  the  film.  Owing  to  the  depth  of  a 
filter  bed,  and  the  zig-zag  course  that  sewage  must  take, 
even  when  flowing  in  large  quantity  through  the  filter,  there 
is  little  or  no  part  of  the  water  and  contained  organic 
matter  which  is  not  brought  into  direct  contact  with  the 
film  on  one  or  more  pieces  of  filtering  material;  and,  when 
the  sewage  is  applied  by  spraying,  the  drops  of  sewage 
trickle  down  from  filtering  piece  to  filtering  piece,  deposit- 
ing on  each  a  portion  of  the  organic  matter  it  carries,  until 
at  last  it  leaves  the  bottom  layer  free  from  the  organic 
matter  it  originally  contained,  but  partly  charged  with 
other  organic  matter  picked  up  in  the  filter  bed,  and 
which,  while  not  entirely  reduced,  is  in  a  non-putrefactive 
condition. 

Owing  to  the  coarseness  of  the  filtering  material  and 
the  manner  in  which  sewage  is  applied,  the  sewage  trickles 
in  thin  films  over  the  surface  of  the  filtering  material  in 
free  contact  with  the  air  which  fills  the  voids.  The  air, 
which  amounts  perhaps  to  five  times  the  volume  of  sewage, 


SEWAGE    PURIFICATION    AND    DISPOSAL  99 

is  able  under  the  most  favorable  circumstances  to  supply 
sufficient  oxygen  for  the  nitrifying  organisms.  It  is  assumed 
that  five  to  ten  volumes  of  air  are  required  for  each  volume 
of  sewage. 

The  process  of  sprinkling  filtration  is  an  aerobic  one 
and  must  be  carried  on  in  the  presence  of  air,  with  a  plenti- 
ful supply  of  oxygen  for  the  reducing  bacteria  to  carry  on 
the  work  of  decomposition  and  nitrification.  The  interstices 
between  the  filtering  material  are  full  of  air  which  is  con- 
stantly replenished  as  fast  as  it  is  consumed  by  the  circu- 
lation of  air  through  the  interstices  of  the  filtering  material, 
and  by  the  application  of  sewage,  which,  falling  like  rain, 
not  only  is  saturated  with  air,  but  carries  more  air  along  by 
momentum  into  the  voids  of  the  filter  bed.  This  plentiful 
supply  of  air  in  a  sprinkling  filter-bed  more  or  less  com- 
pletely oxidizes  the  organic  matter  without  liquefying  or 
gasifying  it;  consequently  the  effluent  from  a  sprinkling  fil- 
ter instead  of  being  clear  like  the  effluent  from  an  inter- 
mittent sand  filter,  contains,  as  a  rule,  a  certain  amount  of 
flocculent  organic  matter.  This  organic  matter,  which  is 
of  a  stable  humus  like  character,  is  carried  in  suspension 
and  can  easily  be  removed  by  sedimentation.  For  the  pur- 
pose of  removing  the  flocculent  matter  from  sprinkling  fil- 
ter effluents,  a  sediment  basin,  through  which  the  effluent 
must  pass,  is  generally  one  of  the  parts  of  a  sprinkling  filter 
plant.  The  suspended  solids  removed  from  sprinkling 
effluents  by  sedimentation  are  sometimes  in  themselves 
putrescible,  but  usually  contain  such  a  small  proportion  of 
unstable  matter  that  they  can  be  discharged  into  a  water- 
course without  producing  a  nuisance.  The  effluents  look 
worse  and  keep  better  than  would  be  expected  from  their 
chemical  composition. 

The  value  of  sprinkling  filters  lies  in  the  fact  that  they 
can  purify  to  a  nonpurificative  stage  a  much  larger  quan- 
tity of  sewage  than  can  be  purified  to  a  like  degree  by  any 
other  process.  The  trickling  effluents,  however,  are  not 
as  pure  as  those  from  intermittent  sand  filters,  although 
they  are  in  general  better  than  those  yielded  by  contact 


100 


SEWAGE    PURIFICATION    AND    DISPOSAL 


beds  or  overworked  sewage  farms.  At  the  present  time, 
sprinkling  filters  are  used  as  a  secondary  or  final  process, 
following  straining  and  septic  action.  It  seems  quite  possi- 
ble, however,  that  in  some  cases,  for  instance  where  sew- 
age has  flowed  a  great  distance  in  sewers,  and  is  there 


Fig.  39 

subjected  to  septic  action,  that  the  crude  sewage  might  be 
applied  to  sprinkling  filters,  after  first  passing  through  a 
screening  chamber  to  remove  the  coarser  solids. 


EXAMPLES  OF  SPRINKLING  FILTERS 

A  sprinkling  filter  consists  of  a  bed  of  coarse  material, 
like  crushed  stone,  coal  or  slag,  placed  on  a  suitably  graded 
impervious  flooring,  that  slopes  toward  a  drain,  so  that  the 
effluent  from  the  sewage  sprayed  onto  the  bed  can  be  col- 
lected and  discharged  at  a  convenient  point.  A  sprinkling 
filter  does  not  require  an  impervious  or  watertight  basin, 
nor  does  it  need  to  be  enclosed  by  walls.  On  the  contrary, 
if  enclosed  in  a  basin,  or  confined  by  walls,  the  masonry 
should  be  laid  up  without  mortar,  and  with  wide  open 


SEWAGE    PURIFICATION    AND    DISPOSAL 


101 


joints  so  that  the  air  can  freely  circulate  through  the  inter- 
stices of  the  filtering-  material. 

Great  diversity  of  design  is  exhibited  in  the  construction 
of  sprinkling  niters.  The  material  of  some  filter  beds  is  left 
unconfined  so  that  the  material  around  the  edges  ultimately 
settles  down  to  the  angle  of  repose.  In  other  beds,  the 
material  is  partly  confined  by  low  stone  or  brick  walls, 
while  in  other  filter  beds  the  material  is  confined  by  hand- 
piling  the  larger  filtering  pieces  around  the  edges,  so  that 
the  bed  will  retain  its  shape  without  settling  to  the  angle  of 
repose. 


s  Sprinkler  Nozzles. 


Fig.  40 


Sewage  is  applied  to  sprinkling  filters  in  a  variety  of 
ways.  In  some  filters,  the  sewage  is  sprayed  from  per- 
forated pipes,  laid  on  the  surface  of  the  filter  bed.  On 
others,  sprinkling  nozzles  are  used,  which  may  be  attached 
to  stand-pipes  rising  from  distributing  mains  buried  in  the 
bed,  or  may  be  connected  to  overhead  pipes,  so  they  will 
spray  downward.  Other  filter  beds  have  revolving  perfor- 
ated arms  which  spray  the  sewage;  while  on  still  others, 
the  sewage  is  distributed  from  traveling  water  wheels 
which  are  actuated  by  the  sewage.  An  example  of  a  rec- 
tangular sprinkling  filter  confined  by  low  walls  is  shown 
in  Fig.  39.  The  underdrain  tiles  rest  on  a  floqr  of  concrete 
which  slopes  toward  a  collector  that  may  be  located  either 
at  one  end  of  the  filter  or  at  the  center.  The  distributing 
mains  are  made  of  cement,  and  are  built  integral  with  the 
floor  on  which  they  rest;  from  the  distributing  mains  at 
suitable  distances  rise  cast  iron  pipes  which  are  surmounted 
by  spraying  nozzles,  through  which  the  sewage  is  sprayed 
onto  the  surface  of  the  bed.  The  sprinkling  filter  shown 


102 


SEWAGE    PURIFICATION    AND    DISPOSAL 


in  this  illustration  is  built  above  ground,  which  is  a  common 
practice. 

When  necessary,  however,  they  may  be  built  below 
the  level  of  the  ground,  but  in  that  case  sufficient  space 
must  be  allowed  at  all  sides  of  the  bed  to  permit  of 
aeration.  Sometimes  walls  enclosing  sprinkling  filters, 


Fig.  41 

particularly  the  dividing  walls  between  niters,  are  built 
solid,  and  tiles  built  into  the  walls  to  admit  air. 

A  sectional  illustration,  giving  the  dimensions  and 
relation  of  the  various  parts  of  a  sprinkling  filter,  is  shown 
in  Fig.  40.  This  illustration  is  only  suggestive,  however, 
as  the  dimensions  and  arrangement  can  be  changed  to  suit 
various  requirements  as  they  may  arise. 

A  battery  of  sprinkling  filters  fitted  with  revolving 
arms  is  shown  in  Fig.  41.  These  beds  are  made  circular  so 
the  spray  will  reach  all  parts  of  the  filtering  material.  The 


SEWAGE    PURIFICATION    AND    DISPOSAL  103 

filtering  material  is  hand-packed  so  the  pile  will  hold  its 
shape,  and  the  entire  bed  rests  on  a  floor  of  bricks  under- 
drained  by  other  bricks  standing  on  edge,  and  radiating 
from  the  center  of  the  bed  to  the  circumference  where  the 
effluent  discharges  freely  into  an  open  gutter.  The  gutter 
is  graded  so  that  the  effluent  can  be  collected  and  discharged 
from  the  lowest  point. 

The  floor  of  a  round  filter  bed  is  generally  made  to 
slope  from  the  center  towards  the  outer  perimeter,  and  is 
covered  with  split-tile,  open  brickwork,  or  some  other  kind 
of  drain  which  will  facilitate  drainage  and  at  the  same  time 
permit  a  free  circulation  of  air. 


SPRINKLING  FILTER  DETAILS 

Materials  for  Sprinkling  Filter  Beds — Considerable  data 
have  been  collected  regarding  the  materials  most  suitable 
for  sprinkling  filters,  and  as  a  general  rule,  derived  from 
the  data,  it  may  be  said  that  any  hard,  durable  material 
will  serve  well  for  this  purpose.  The  chief  requisite  is 
that  the  material  be  of  a  stable  character  which  will  not 
soften  or  disintegrate  under  the  action  of  water.  Anthra- 
cite coal  has  been  found  particularly  suitable  for  this  pur- 
pose, but  on  account  of  the  cost  of  coal,  it  is  not  extensively 
used.  Coke,  slag  and  furnace  cinders,  each  has  been  suc- 
cessfully used,  and  in  many  localities  are  preferable  to 
other  material  on  account  of  their  low  cost.  Granite,  trap 
rock,  flints,  gravel,  hard  clinker,  slate,  blast  furnace  slag, 
boiler  slag  and  broken  bricks,  have  all  been  successfully 
employed  for  sprinkling  filter  beds.  Smooth  material  is 
not  so  good  as  rough  material  when  a  filter  bed  is  first  put 
into  service,  but  the  difference  soon  disappears  as  the  filters 
grow  old,  after  which  time  no  difference  in  efficiency  can 
be  detected.  As  a  rule,  the  material  to  use  for  a  sprinkling 
filter  bed  will  depend  on  the  availability  of  the  several 
materials  in  the  locality  where  the  plant  is  to  be  con- 
structed. There  is  hardly  a  part  of  the  civilized  world 
where  some  one  of  the  suitable  materials  is  not  at  hand, 


104  SEWAGE    PURIFICATION    AND    DISPOSAL 

and  can  be  had  at  low  cost;  and  the  cost  of  the  material 
will  generally  determine  the  choice. 

Size  of  Material  for  Sprinkling  Filter  Beds— The  size 
of  material  for  sprinkling  filter  beds  may  vary  considerably 
within  certain  limits,  but  the  various  sizes  of  filtering 
material  within  any  one  bed  should  be  proportioned  to  one 
another.  For  instance,  the  size  of  materials  may  vary  from 
/s  inch  to  2}4  inches  but  those  extreme  sizes  should  never 
be  placed  in  the  same  bed.  Ordinarily,  the  extreme  pro- 
portion or  ratio  should  be  as  one  to  four.  That  is,  in  a  filter 
bed  in  which  the  smallest  size  material  is  ^6 -inch,  the  largest 
size  should  not  be  over  ^2 -inch,  while  in  a  filter  bed  in 
which  the  smallest  size  of  material  is  ^-inch,  the  largest 
should  not  exceed  2  inches.  The  usual  sizes  of  filtering 
material  that  go  together  are  ^-inch  to  ^-inch;  ^-i 
to  24 -inch;  ^6-inch  to  i-inch;  ^-inch  to  i^ -inches; 
to  2-inches;  i-inch  to  2  ^2 -inches. 

The  sizes  of  stones  for  filtering  material  are  determined 
by  the  size  mesh  of  screen  they  will  pass  through;  for 
instance,  a  ^-inch  filtering  material  is  a  material  which 
will  pass  through  a  screen  of  ^-inch  mesh,  and  will  be 
held  back  by  a  screen  of  y/s -inch  mesh.  When  two  sizes  of 
stones  are  specified  for  a  filtering  material,  pieces  of  just 
those  two  sizes  are  not  intended,  but  any  size  falling  be- 
tween those  two  are  included.  For  instance,  material  from 
^3 -inch  to  ^-inch  means  all  the  material  which  will  pass 
through  a  ^-inch  screen  and  will  be  held  back  by  a  -^L-inch 
screen.  It  may  safely  be  assumed,  that  the  smaller  the 
medium  the  more  complete  will  be  the  purification  obtained 
for  beds  of  equal  depth,  but  when  the  beds  are  propor- 
tioned in  depth  to  the  size  of  material  there  is  but  little 
difference  between  the'  efHuent  obtained  from  beds  with 
different  size  materials ;  and  when  treating  a  like  sewage  but 
little  better  result  can  be  obtained  by  the  use  of  ^6 -inch 
to  ^-inch  material  than  can  be  obtained  by  ^-inch  to  1^2- 
inch  material.  There  are  other  conditions,  however,  which 
determine  the  size  of  material  most  suitable  to  tise.  For 
instance,  if  the  filtering  material  is  softer  than  is  desired, 


SEWAGE    PURIFICATION    AND    DISPOSAL  105 

but  is  the  best  material  available  at  a  reasonable  cost,  the 
better  practice  no  doubt  would  be  to  use  large  sizes  to 
prevent  the  bed  clogging  from  the  disintegrating  portion. 

The  power  of  coarse  sprinkling  filters  to  unload  stored 
material  in  a  condition  so  that  it  can  be  easily  disposed  of 
on  land  without  creating  a  nuisance  is  of  much  practical 
value  when  treating  a  strong  sewage,  and  is  a  feature  that 
will  recommend  coarse  filters  to  most  engineers. 

Large  material  is  likewise  used  when  constructing  a 
filter  to  care  for  an  exceptionally  large  quantity  of  sewage, 
as  large  materials  permit  a  freer  vertical  circulation  of  air. 
The  larger  sizes  of  material  are  less  liable  to  clog  than 
would  the  smaller  sizes,  and  the  larger  sizes  will  permit 
better  aeration  of  the  bed  than  would  fine  filtering  materials. 
Fine  filtering  material,  on  the  other  hand,  is  preferable 
when  sewage  is  to  be  applied  at  a  moderate  rate  and  a 
better  effluent  is  desired. 

Depth  of  Sprinkling  Filter  Beds— The  depth  of  filtering 
materials  for  a  sprinkling  filter  depends  somewhat  on  the 
degree  of  purification  desired.  It  might  be  said  that  the 
purification  effected  is  proportioned  to  the  depth  of  the  bed, 
but  not  in  direct  proportion,  nor  is  the  additional  purifica- 
tion sufficient  to  warrant  extending  a  bed  to  a  very  great 
depth.  The  minimum  depth  for  a  sprinkling  filter  bed  is 
perhaps  four  feet,  but  a  greater  depth  is  desirable  on  ac- 
count of  the  danger  of  raw  sewage  passing  through  chan- 
nels in  shallow  filters  due  to  irregular  packing  of  the 
material.  On  the  other  hand,  a  sprinkling  filter  bed  seldom 
need  be  made  deeper  than  ten  feet.  The  purification 
effected  in  any  additional  depth  would  hardly  be  sufficient 
to  warrant  the  expense.  As  a  basis  for  estimating  the  depth 
of  the  filter  beds  it  might  be  assumed  that  five  feet  will  be 
sufficient  when  the  effluent  discharges  into  tide  water,  or 
into  the  lower  reaches  of  a  river  below  the  intake  to  a 
waterworks  pumping  station ;  eight  feet  where  the  effluent 
discharges  into  a  river  having  a  dry  weather  flow  of  over  45 
times  the  volume  of  sewage,  and  ten  feet  where  the  effluent 
discharges  into  smaller  streams. 


106  SEWAGE    PURIFICATION    AND    DISPOSAL 

Period  of  Flow  Through  Sprinkling  Filters— The  period 
of  flow  through  sprinkling  filter  beds  varies  from  two  or 
three  minutes  to  thirty  minutes.  In  beds  made  of  large 
materials,  seldom  more  than  three  minutes  are  consumed  in 
passing  through  the  bed,  while  in  materials  ranging  in  size 
from  ^6-inch  to  ^-inch,  thirty  minutes  is  not  an  unusual 
period. 

DISTRIBUTORS   FOR  SPRINKLING  FILTERS 

Fixed  Sprinklers — The  ideal  method  for  the  distribu- 
tion of  sewage  on  a  sprinkling  filter  would  be  to  discharge 
it  continuously  in  a  fine  even  spray  over  the  entire  surface 
of  the  filter.  The  sewage  being  finely  divided  in  the  form 
of  spray  would  have  every  particle  exposed  to  the  atmo- 
sphere and  would  enter  the  voids  of  the  filter  bed  well 
charged  with  air,  besides  carrying  along  more  by  its 
momentum. 

The  ideal,  however,  is  never  attained  in  practice. 
With  revolving  sprinklers  and  traveling  distributors  the 
application  is  intermittent  instead  of  continuous;  and  with 
fixed  sprinklers,  owing  to  certain  limitations,  the  entire 
bed  can  never  be  sprayed.  When  fixed  sprinklers  are 
used,  each  sprinkler  nozzle  occupies  the  center  of  an  imagi- 
nary square  on  the  surface  of  the  filter  bed,  and,  as  the 
spray  from  the  sprinkler's  nozzle  falls  in  a  circle,  the  four 
corners  of  the  square  will  remain  unwetted.  The  area  of 
a  circle  inscribed  in  a  square  being  only  .7854  of  the  area 
of  the  square,  it  follows,  theoretically,  that  78  per  cent,  of 
the  surface  is  all  that  can  be  covered  by  a  fixed  sprinkler 
nozzle.  As  a  matter  of  fact,  however,  the  percentage  is 
somewhat  increased  by  staggering  the  sprinkler  nozzle,  so 
that  instead  of  having  a  square  surface  to  cover,  each 
sprinkler  is  set  in  the  center  of  an  irregular  surface, 
bounded  by  the  spray  from  the  other  sprinkler  heads,  so 
that  actually  about  92  per  cent,  of  the  alloted  surface  is 
covered  when  the  sprinklers  are  spaced  at  the  correct  dis- 
tance for  the  heads  they  are  operated  under. 

Of  the  surface  covered  by  spray  from  fixed  sprinkers, 


SEWAGE    PURIFICATION    AND    DISPOSAL  107 

the  distribution  is  not  uniform  with  all  types  of  sprinkler 
heads,  but,  on  the  contrary,  varies  at  different  distances 
from  the  sprinkler  nozzle;  so  that  if  the  surface  covered 
by  the  spray  be  divided  into  a  series  of  concentric  circles, 
some  of  the  circles  will  receive  more  spray  than  others. 
This  is  not  true  of  all  sprinkler  heads.  On  the  contrary, 
the  new  Salford  nozzle,  also  the  Birmingham  nozzle,  effect 
an  almost  perfect  distribution  of  sewage  over  the  surface 
wetted.  The  openings  to  these  nozzles  are  so  small,  how- 
ever, that  they  are  liable  to  cause  trouble  from  clogging 
when  coarse  sewage  is  used.  The  openings  to  the  Salford 
nozzle  are  only  /^-inch  in  diameter,  while  that  of  the 
Birmingham  nozzle  is  an  annular  ring  only  g5¥-inch  wide. 
In  the  American  types  of  sprinkler  heads,  like  the  Colum- 
bus, in  which  the  openings  are  made  larger,  the  distribu- 
tion is  much  less  uniform.  This  objection  may  be  over- 
come, howrever,  by  varying  the  head  on  the  nozzle  so  that 
the  points  of  maximum  or  excessive  wetting  will  be  con- 
stantly changing,  thus  making  the  distribution  over  the 
entire  surface  almost  uniform.  The  head  may  be  varied 
on  the  sprinkler  nozzles  either  by  dosing  from  duplicate 
tanks,  which  are  filled  and  discharged  alternately,  or  by 
means  of  automatic  siphons.  It  is  found  by  experiment 
that  with  varying  heads  the  distribution  is  better  when  the 
tanks  are  not  too  long  in  emptying,  since  with  the  low 
heads,  that  is,  anything  less  than  three  feet,  the  distribu- 
tion is  not  so  good  as  with  the  greater  heads. 

Wind  is  another  factor  which  interferes  with  the 
uniform  distribution  from  sprinkling  filters.  This  feature, 
however,  can  be  overcome  or  the  effect  minimized  only  by 
selecting  a  site  where  the  prevailing  winds  are  not  strong 
or,  in  extreme  cases,  by  constructing  a  wind-break. 

The  method  of  applying  sewage  to  sprinkling  niters 
varies  in  different  countries,  preference  being  given  in 
America  to  distribution  by  fixed  sprinklers,  while  in  Great 
Britain  revolving  sprinklers  and  traveling  distributors  seem 
to  have  the  preference. 

A  system  of  fixed  sprinklers  for  a  rectangular  filter  bed 


108 


SEWAGE    PURIFICATION    AND    DISPOSAL 


is  shown,  both  in  plan  and  in  section,  in  Fig".   42.     In  a 
system  of  this  kind,  each  distributing  main  is  controlled  by 


•Sect/on  fhrouqh  F//fer  Bed'. 


llector. 
sMa//7  Drain: 


•r 


Fig.  43 


P/an  of  F/tter  Dtsfr/'buf/nei 


SEWAGE    PURIFICATION    AND    DISPOSAL 


109 


a  valve  located  in  a  gate  house,  so  that  sewage  can  be  shut 
off  from  that  part  of  the  filter  supplied  by  the  main. 

The  distributing  mains  may  be  made  of  cement  con- 
crete, or  may  be  of  iron.  In  the  latter  case,  cast-iron  pipe 
should  be  used,  as  wrought-iron  and  steel  pipe  have  com- 
paratively short  lives  when  used  in  such  manner.  Gener- 
ally, the  lateral  distributors  are  made  of  earthernware  pipe 
embedded  in  concrete.  It  is  a  good  practice  to  provide 
means  for  flushing  the  distributing  mains  to  remove  the 
deposits  which  accu- 
mulate therein,  due  to 
the  depository  veloci- 
ties maintained  to  re- 
duce the  head  that 
otherwise  would  be 
lost  by  friction. 

Instead  of  burying 
the  main  distributors 
near  the  bottom  of  the 
filter  bed,  in  some 
plants  the  mains  are 
laid  on  top  of  the  bed 
and  the  sprinklers  fit- 
ted, hand  tight,  into 
the  holes  of  bosses 
made  specially  for  this 
purpose.  In  other 
plants  the  mains  are 
buried  about  one  foot  from  the  surface,  while  in  others, 
which  are  more  economically  built,  and  designed  to  operate 
under  a  lower  head,  perforated  salt-glazed  pipe  distribu- 
tors are  laid  along  the  surface  of  the  beds,  about  six  feet 
apart;  the  tile  pipes  are  from  3  to  4  inches  in  diameter  and 
each  length  is  perforated  with  two  ^  or  ^6 -inch  holes, 
through  which  the  sewage  is  sprayed  onto  the  beds. 

It  will  be  noticed  that  the  vertical  branches  from  the 
lateral  distributors  to  the  nozzles  are  not  tapped  in  a  direct 
line  across  from  one  another,  but,  on  the  contrary,  are 


110 


SEWAGE    PURIFICATION    AND    DISPOSAL 


staggered.  This  is  done  to  obtain  a  better  distribution  of 
the  sewage  than  would  be  effected  if  the  outlets  were  all  in 
line.  A  reference  to  Fig.  43  will  show  that  when  the 
sprinklers  are  staggered,  there  is  but  little  of  the  surface 
which  receives  a  double  dose  of  sewage,  and  very  little 
surface  to  which  sewage  is  not  applied;  so  little,  in  fact, 
that,  owing  to  splashing,  action  of  the  wind,  and  lateral 
movement  of  the  sewage  through  the  filtering  material, 
within  a  few  inches  below  the  surface  of  the  bed  there 
would  doubtless  be  a  fairly  uniform  flow  of  sewage  over 
the  filtering  material.  In  the  case  of  Fig.  44,  however,  the 

result  is  different. 
Here  the  sprinklers 
are  all  in  line,  and, 
spaced  equal  distances 
apart  and  with  the 
same  diameter  of  spray 
as  used  when  the  pipes 
are  staggered,  there 
would  be  parts,  a,  a, 
of  the  filter  which 
would  not  be  sprayed, 
while  other  parts,  b,  b, 
would  receive  a  double 
dose.  If  a  less  diam- 
eter of  spray  be  adopted  in  this  case,  while  little  or  no  sur- 
face would  receive  a  double  dose  of  sewage,  the  parts 
actually  sprayed  would  be  reduced  to  such  an  extent  that  a 
large  part  of  the  filter  surface  would  be  useless. 

The  distance  apart  that  sprinklers  can  be  spaced 
depends  entirely  upon  the  available  head  of  sewage  and 
the  volume  of  discharge.  The  lowest  workable  head  for 
the  average  sprinkler  nozzle  is  2  feet  9  inches,  and  the  most 
effective  head  about  6  feet.  Nozzles  differ  so,  in  many 
respects,  that  in  each  case  the  type  to  be  used  should  be 
tested  to  find  out  the  diameter  of  spray  and  volume  dis- 
charged for  each  foot  of  head.  Ordinarily,  for  each  foot 
of  head  available,  the  average  jet  will  cover  a  surface 


-L  . 

.L    .. 

L 

A/ 

/  - 

^6      !•) 

>;H  x> 

/  Y*-°-\- 
<£><•> 

'               \  ' 

3-^v- 

•    H        N     / 

.?  1        v 

-< 

A 

'^i  '          y 

/    \ 

i.  1        II 

» 

\    i 

\  \ 

/ 

\  i 

i                       y 

Y 

/ 

i    yv 

/\ 

1       /    V 

/ 

Fig.  44 


SEWAGE    PURIFICATION    AND    DISPOSAL  111 

approximately  2  feet  in  diameter,  consequently,  for  rough 
calculation,  it  may  be  assumed  that,  with  a  4^2 -foot  head, 
the  sprinklers  may  be  spaced  9  feet  apart;  with  a  5 -foot 
head,  10  feet  apart;  with  a  5^ -foot  head,  n  feet  apart, 
and  with  a  6-foot  head,  12  feet  apart,  provided  the  quantity 
of  sewage  discharged  is  proportioned  to  the  area  covered. 
There  is  a  certain  intimate  relation  between  the  quantity 
of  sewage  discharged  from  a  sprinkler  nozzle,  the  head 
required  to  produce  the  discharge  and  the  area  of  filter 
surface  covered  which  must  not  be  overlooked.  For  in- 
stance, in  the  American  types  of  sprinkler  nozzles,  the 
openings  are  made  large  to  prevent  stoppage.  In  the 
Columbus  sprinkler  nozzle,  operating  under  a  4.3-foot 
head,  the  discharge  is  10.9  gallons  per  minute.  At  this 
rate  of  discharge,  131  sprinkler  nozzles  are  required  per 
acre  to  equal  a  rate  of  filtration  of  2,000,000  gallons  per 
24  hours.  At  this  rate  of  filtration  the  sprinklers  would 
have  to  be  spaced  19  feet  apart,  but  the  nozzle  operating 
under  a  4.3-foot  head  would  cover  an  area  only  12  feet  in 
diameter,  so  that  while  the  filter  bed  would  be  treating 
sewage  at  the  rate  of  2,000,000  gallons  per  24  hours,  the 
sewage  would  be  treated  on  only  14,816  square  feet  of 
surface  out  of  43,560  square  feet  in  an  acre,  or  on 
about  one-third  of  the  surface,  which  would  equal  a 
rate  of  about  6,000,000  gallons  per  24  hours.  To 
overcome  this,  on  some  filters  a  head  of  7^  feet  is  main- 
tained on  the  sprinkler  nozzles,  or  a  variable  head  alternat- 
ing from  7^2  feet,  with  a  discharge  of  13.5  gallons  per 
minute,  to  a  head  of  4.3,  with  a  discharge  of  10.9  gallons 
per  minute.  At  a  rate  of  discharge  of  13.5  gallons  per 
minute,  the  nozzles  can  be  spaced  16.2  feet  apart  and  the 
surface  between  them  well  wetted  with  sewage.  Spacing 
the  nozzles  16.2  feet  apart  would  require  180  nozzles  to  an 
acre,  and  at  a  flow  of  13.5  gallons  per  nozzle,  would  be 
equal  to  a  filtration  capacity  of  about  3,500,000  gallons  per 
acre  per  day.  This  quantity,  however,  would  be  sprinkled 
on  about  92  per  cent,  of  the  filter  area,  which  would 
increase  the  actual  rate  of  filtration  through  the  surface 


112 


SEWAGE    PURIFICATION    AND    DISPOSAL 


Figr.  45 


wetted  to  approximately  4,000,000  gallons  per  acre  per  24 
hours.  Where  plants  are  designed  for  this  rate  of  nitra- 
tion, the  nozzles  are  in  operation  only  one-half  the  time, 
so  that,  while  the  rate  of  nitration  while  in  operation  is 
4,000,000  gallons  per  acre  per  day,  the  actual  quantity  fil- 
tered is  only  about  2,000,000  gallons  per  acre  per  day.  To 

secure  an  actual  rate  of  filtration 
of  2,000,000  gallons  per  acre  per 
24  hours,  actual  operation,  while 
spraying  continuously,  the  noz- 
zles can  be  operated  under  a  vari- 
able head  from  4  feet  to  2  feet. 
This  gives  a  discharge  of  approxi- 
mately 4  gallons  per  minute,  dis- 
tributed over  an  area  of  10^2  feet 
in  diameter,  and  permits  spacing 
the  nozzles  about  n  feet  apart,  which  is  equivalent  to 
about  390  nozzles  per  acre.  Sixteen  feet  two  inches  is  the 
greatest  distance  that  sprinklers  are  spaced  in  the  present 
state  of  sewage  purification  practice. 

Sprinkler  Nozzles — A  sprinkler  nozzle  which  is  both 
simple  and  effective,  and 
which  will  work  under  a  low 
head,  is  shown  in  Fig.  45. 
The  nozzle,  which  is  known  as 
the  Columbus,  consists  of  a 
single  orifice,  T9¥-inch  in  diam- 
eter, above  which  an  inverted 
cone  is  held  by  two  thin  arms, 
the  axis  of  the  cone  coincid- 
ing with  the  axis  of  the  ori- 
fice. The  jet  upon  leaving 
the  orifice  impinges  against  the  cone  and  is  transformed 
into  a  thin  sheet,  which  spreads  out  radially  and  then  breaks 
into  a  mass  of  drops,  which  fall  on  an  area  included  within 
a  circle  of  a  diameter  depending  on  the  head.  The  princi- 
pal objection  to  this  type  of  nozzle  lies  in  the  fact  that 
the  two  arms  which  hold  the  inverted  cone  in  place, 


Fig.  46 


SEWAGE    PURIFICATION    AND    DISPOSAL 


113 


theoretically,  protect  a  certain  part  of  the  bed  from  being 
sprayed  with  sewage;  as  a  matter  of  fact,  however,  the 
blades  or  arms  are  so  thin  at  their  edges  that  the  sheet  of 
water,  after  having  been  separated,  closes  up  again  after 
passing  the  arms,  so  that  sewage  sprinkled  from  such  a 
nozzle  will  cover  the  entire  surface  included  within  its 
range.  However,  a  modified  design  of  this  type  of  sprinkler 
head  is  now  made  which  does  away  with  that  obiection. 
This  sprinkler  head  is  shown  in  Fig.  46.  It  consists  of  an 
inverted  frustum  of  a  cone,  carried  by  a  rod  passing  through 
the  center  of  a  -^-J-inch  nozzle  orifice.  The  cone  slopes  45 
degrees,  is  i^  inches  in  diameter  across  the  top,  and  is 


Fig.  47 

carried  by  a  T5^-inch  rod.  The  junction  of  the  rod  and  the 
cone  is  -^-inch  above  the  plane  of  the  orifice.  To  operate 
satisfactorily  the  rod  must  be  kept  exactly  in  the  center 
of  the  orifice. 

Proportioning  Distributing  Systems — Larger  pipes  are 
allowed  in  designing  the  distributing  system  for  a  sprink- 
ling filter  than  would  be  required  under  different  conditions 
to  conduct  equal  quantities  of  water  or  sewage.  The  extra 
capacity  of  pipes  is  allowed  to  reduce  to  the  minimum  the 
loss  of  head  from  friction,  compensate  for  the  reduction  in 
size  due  to  tuberculation  and  sedimentation,  and  to  provide 
a  large  volume  of  sewage  to  maintain  a  high  temperature, 
thus  preventing  the  sewage  from  freezing.  In  most  of  the 


114 


SEWAGE    PURIFICATION    AND    DISPOSAL 


large  sprinkling  filters  now  in  use,  3-inch  cast-iron  pipes 
are  used  as  risers  to  supply  the  sprinkler  heads,  which  dis- 
charge sewage  at  the  rate  of  from  10  gallons  per  minute, 
when  spraying  at  the  minimum  rate,  up  to  13.5  gallons  per 
minute,  when  operating  at  the  maximum  rate. 

Revolving  Sprinklers — Revolving  sprinklers  are  used 

principally  in  Great 
Britain.  They  con- 
sist simply  of  radial 
perforated  tubes, 
which  are  caused  to 
revolve  around  a  cen- 
ter axis  by  means  of 
the  sewage  flowing 
through  the  pipes 
and  acting  on  the 
vanes  of  a  water 
wheel.  The  con- 
struction of  a  revolv- 
ing  sprinkler  is 
shown  in  perspective 
in  Fig.  47,  and  the 
manner  in  which  it 
operates  is  illustrated 
in  Fig.  48.  The 
weight  of  the  appa- 
ratus is  supported  by 
the  upright,  «,  and 
the  movable  part  re- 
volves on  ball  bear- 
ings, £,  located  in  the 
head  casing.  Sewage 
is  conducted  into  the  distributor  through  the  pipe,  r,  and 
issues  from  the  central  pillar,  </,  through  fixed  pipes,  e,  e. 
As  the  sewage  flows  from  the  mouths  of  these  pipes,  it 
impinges  against  the  blades,  /,  f.  which  are  attached  to  the 
trough,  and  gives  the  arms  an  impulse  to  the  left.  The 
sewage  then  falls  to  the  bottom  of  the  trough,  passes  into 


Fig.  48 


SEWAGE    PURIFICATION    AND    DISPOSAL 


115 


the  four  radial  arms,  g,  g,  and,  issuing  from  perforations, 
//,  in  the  right  side  of  the  arms,  gives  the  sprinkler  a  further 
impulse,  the  combined  efforts  of  which  cause  the  sprinkler 
to  revolve.  These  sprinklers  are  made  with  two,  four  and 
five  arms,  and  are  designed  to  distribute  sewage  at  the  rate 
of  200  gallons  per  square  yard  per  day  of  24  hours'  contin- 
uous flow.  Sprinklers  of  two  arms  are  used  for  beds  of  60 
feet  diameter  and  smaller,  while  four  and  five-arm  sprink- 
lers are  used  for  beds  of  larger  diameters.  Revolving 
sprinklers  have  been  successfully  used  in  England  on  beds 
130  feet  in  diameter. 

Revolving  sprinklers  require  considerable  attention  to 


Fig.  49 

keep  them  in  working  order,  daily  .cleaning  with  brushes 
sometimes  being  necessary  to  prevent  the  holes  from  being 
clogged. 

Automatic  Traveling  Distributor — A  sewage  distributor 
for  a  rectangular  bed,  which  distributes  sewage  by  splash- 
ing or  emptying  it  from  the  buckets  of  traveling  water 
wheels,  is  shown  in  Fig.  49.  This  distributor  receives 
sewage  through  pipe  a  from  the  trough  £,  which  extends 
the  entire  length  of  the  filter  bed.  From  the  pipe  a  the 
sewage  flows  into  pipe  c,  or  into  a  similar  pipe  on  the  oppo- 
site side  of  the  distributor,  depending  on  which  way  the 
apparatus  is  traveling.  Assuming  that  the  apparatus  is 
approaching,  the  flow  of  sewage  would  be  through  pipe  c 


116 


SEWAGE    PURIFICATION    AND    DISPOSAL 


and  spouts  </,  d,  into  the  buckets  of  each  alternate  wheel 
e,  e.  The  weight  of  sewage  in  these  buckets  causes  the 
wheels  to  revolve  and  the  distributor  to  travel  along. 
When  the  distributor  reaches  a  certain  point,  the  valve 
stem  f  strikes  a  tripping  device,  which  shuts  off  the  flow 
of  sewage  from  pipe  r,  and  shunts  it  into  the  similar  pipe 
on  the  opposite  side.  From  this  pipe,  the  sewage  overflows 
through  spouts  into  the  buckets  of  wheels  g^  g,  causing 
them  to  revolve  in  the  opposite  direction,  carrying  the 
entire  apparatus  along.  It  will  be  observed  that  the  appli- 


Fig.  50 

cation  of  sewage  by  means  of  this  apparatus  is  intermittent 
instead  of  continuous,  and  that  sewage  is  applied  to  only 
half  the  width  of  the  bed  when  the  apparatus  is  moving 
along. 

Automatic  Revolving  Distributors — The  splash  system 
of  distribution  is  likewise  adapted  to  circular  beds,  as 
shown  in  Fig.  50.  The  only  difference  is  that  instead  of  a 
reciprocating  movement,  the  water  wheels  travel  continu- 
ously in  a  circle.  In  this  manner,  while  the  application  of 
sewage  is  intermittent,  the  entire  width  of  the  filter  bed  is 
wetted  while  the  water  wheel  is  traveling  over  it. 


SEWAGE    PURIFICATION    AND    DISPOSAL  117 

Capacity  of  Sprinkling  Filters — The  rate  at  which 
sprinkling  niters  can  be  operated  seems  to  lie  somewhere 
between  one  million  gallons  per  acre  per  day  and  three 
million  gallons  per  acre  per  day.  Perhaps  two  million 
gallons  per  acre  per  day  is  a  fair  average  when  the  filters  are 
working  continuously,  or  at  the  rate  of  four  million  gallons 
per  twenty- four  hours  when  resting  half  the  time.  The  rate 
of  nitration  that  can  successfully  be  used  depends  consider- 
ably on  the  strength  of  the  applied  sewage.  When  treating 
an  ordinary  American  sewage  containing  much  street 
refuse,  a  rate  of  two  million  gallons  per  acre  per  day  can  be 
safely  adopted.  When  treating  sewage  from  a  city  having 
a  separate  system  of  sewers,  or  when  treating  septic  effluent 
from  country  institutions,  the  septic  effluent  can  be  applied 
at  the  rate  of  two  and  one-half  million  gallons  per  acre  per 
day,  which  is  equal  to  a  rate  of  about  500  gallons  per  square 
yard  per  day.  Allowing  a  per  capita  consumption  of  water 
of  125  gallons  per  day,  one  acre  of  filter  surface  at  that  rate 
would  care  for  the  sewage  from  a  city  of  20,000  population. 

Efficiency  of  Sprinkling  Filters — Sprinkling  filters  are 
not  a  substitute  for  intermittent  sand  filters,  which  remove 
practically  all  the  matter  in  suspension  in  the  sewage,  but 
are  simply  devices  for  the  quick  oxidation  of  the  putrefying 
matters  in  sewage,  while  allowing  the  larger  body  of  stable 
or  slowly  decomposing  matters  to  pass  along  with  the  efflu- 
ent. The  purification  effected  by  sprinkling  filters  is  good, 
and  would  be  considered  better  in  general  than  that  obtained 
by  the  double  contact  process.  The  effluent  from  a  sprink- 
ling filter  is  likewise  better  than  those  yielded  by  single  con- 
tact beds  and  sewage  farms  which  are  overworked,  but  is 
not  so  good  as  that  produced  by  intermittent  filtration. 
The  effluent  from  sprinkling  filters  contain  as  a  rule  a  certain 
amount  of  flocculent  organic  matter  which  mars  its  appear- 
ance, but  this  matter  has  been  more  or  less  oxidized  and  is 
of  a  stable,  humus-like  character.  The  suspended  solids  in 
sprinkling  effluents  can  easily  be  removed  by  a  short  sedi- 
mentation. It  may  safely  be  assumed  that  sprinkling 
filters  operated  at  proper  rates  for  the  strength  of  sewage 


118  SEWAGE    PURIFICATION    AND    DISPOSAL 

applied  will  produce  effluents  containing  nitrates  averaging 
two  parts  per  100,000.  The  effluent  from  the  filters  will 
contain  approximately  50  per  cent,  of  the  organic  matter  in 
the  applied  sewage,  but  this  organic  matter  will  be  freed 
from  most  of  its  easily  putrescible  bodies,  so  that  the  resi- 
due of  organic  matter  will  be  stable.  There  is  a  visible 
difference  between  the  effluents  from  coarse-grained  and 
fine-grained  sprinkling  filters.  The  effluents  from  coarse- 
grained filters  are  not  clear,  but  contain  large  amounts  of 
suspended  flocculent  matter,  while  the  effluents  from  fine- 
grained filters  may  be  clear.  The  effluents  from  sprinkling 
filters,  either  coarse-grained  or  fine-grained,  are  non-putres- 
cible,  but  bacteriologically,  they  are  not  much  better  than 
the  crude  sewage,  and  are  not  good  enough  to  be  turned 
into  streams  which  are  used  without  filtration  for  water 
supply,  or  into  bays  or  tidal  estuaries  where  oyster  beds 
are  located. 

Settling  Basins — Settling  basins  are  usually  provided 
to  remove  from  sprinkling-filter  effluents  the  suspended 
organic  matter  which  is  of  a  fairly  coarse  nature.  These 
basins  may  be  made  in  any  desired  shape,  should  have  a 
capacity  of  from  two  to  three  hours'  flow,  and  should  have 
a  depth  of  about  ten  feet.  The  bottom  of  the  basin  should 
slope  toward  a  sludge  outlet  to  facilitate  the  removal  of 
sludge.  Effluent  should  flow  through  the  settling  chambers 
at  a  very  low  velocity,  probably  at  a  rate  not  greater  than 
two  inches  per  minute,  to  allow  time  for  the  sludge  to  settle. 
About  one  cubic  yard  of  sludge  will  accumulate  in  settling 
basins  for  each  1,000,000  gallons  of  effluent  passing 
through.  Shallow  tanks  are  sometimes  successfully  used, 
and  the  present  tendency  of  practice  is  to  interpose  a 
screen  composed  of  two  wire  nettings  about  12  inches 
apart  and  filled  with  coke,  between  the  inlet  and  outlet  to 
the  tank. 

Odors  from  Sprinkling  Filters — Where  sprinkling  filters 
are  properly  designed,  constructed  and  operated,  no  objec- 
tionable odors  should  be  noticeable  beyond  one-fourth  mile 
from  large  plants,  and  usually  not  beyond  a  few  hundred 


SEWAGE    PURIFICATION    AND    DISPOSAL  110 

feet.  The  odors  from  small  plants  will  be  noticeable  at 
correspondingly  less  distances. 

Permanency  of  Sprinkling  Filter  Beds — The  filtering 
material  in  sprinkling  filter  beds  will  probably  require 
removing,  cleaning  and  replacing  once  in  from  ten  to  fifteen 
years.  Occasionally,  however,  the  underdrains  may  require 
flushing  to  wash  out  deposits  of  organic  matter,  and  deposits 
might  sometimes  accumulate  in  the  distributing  mains 
or  branches. 

Effect  of  Temperature  on  Sprinkling  Filters — No  data 
outside  of  experimental  data,  are  obtainable  showing  the 
effect  of  severe  winter  weather  on  sprinkling  filters.  In 
the  absence  of  definite  knowledge  it  may  safely  be  assumed 
that  sprinkling  filters  cannot  successfully  be  used  in  locali- 
ties having  a  mean  winter  temperature  below  twenty  de- 
grees Fahrenheit. 

CONTACT  BEDS 

Principles  of  the  Contact  Bed — If  sewage  be  discharged 
into  a  watertight  tank,  which  is  filled  with  broken  stone, 
coal,  glass,  coke  or  other  material ;  is  allowed  to  stand 
there  in  contact  with  the  filling  material  for  a  certain  period 
of  time,  and  the  effluent  is  then  withdrawn  so  the.  inter- 
stices of  the  material  can  fill  with  air,  it  will  be  found  that 
a  material  purification  has  been  effected  in  the  sewage, 
from  which  a  considerable  portion  of  the  organic  matter 
has  disappeared,  while  the  residue,  which  passes  off  with 
the  effluent,  has  been  freed  from  most  of  its  easily  putrefied 
compounds,  so  that  it  is  of  sufficient  stable  quality  to  create 
no  nuisance. 

The  filling  material  in  a  contact  bed  is  covered  with  a 
jelly-like  film,  which  forms  a  habitat  for  the  reducing 
and  nitrifying  micro-organisms,  and  the  filling  material 
helps  to  entangle  organic  matter  in  these  films  by  the  attrac- 
tive power  the  particles  exert  on  the  suspended  matter  in 
the  sewage.  The  length  of  time  that  the  sewage  is  in  con- 
tact with  the  filling  material  is  an  important  consideration. 
If  the  period  be  too  short,  sufficient  purification  will  not 


120  SEWAGE    PURIFICATION    AND    DISPOSAL 

take  place,  while  if  the  period  be  prolonged  beyond  the 
proper  stage,  putrefaction  will  begin,  producing  a  dark, 
disagreeable-smelling  effluent,  which  is  difficult  to  purify. 

The  process  which  takes  place  in  a  contact  bed  is  evi- 
dently a  very  complex  one,  where  either  aerobic  and  anae- 
robic processes  take  place  alternately  or  they  are  carried 
on  simultaneously  side  by  side.  During  the  process  the 
nitrates  formed  while  the  bed  is  empty  are  partly  or  wholly 
consumed  during  the  period  of  contact,  and  for  this  reason 
the  nitrates  found  in  the  final  effluent  from  a  contact  bed 
are  not  a  true  measure  of  the  purification  effected,  since 
under  favorable  conditions  the  nitrates  formed  from  half 
the  nitrogen  would  be  used  up  in  decomposing  the 
other  half. 

To  be  successfully  operated,  a  contact  bed  should  be 
allowed  sufficiently  long  and  frequent  periods  of  rest,  and 
the  sewage  applied  to  the  beds  should  be  of  uniform  char- 
acter and  free  as  possible  from  suspended  matter.  On  this 
account,  some  form  of  preliminary  treatment  is  advisable 
before  applying  sewage  to  contact  beds,  particularly  when 
only  single  contact  is  provided  for.  Septic  tanks,  or  sedi- 
ment tanks,  are  the  preliminary  treatment  commonly  re- 
sorted to  in  connection  with  contact  beds.  The  treatment 
of  septic  effluent  instead  of  crude  sewage  greatly  prolongs 
the  life  of  a  contact  bed,  while  at  the  same  time  the  loss  of 
water  capacity  is  less,  and  consequently  the  capacity  of  the 
contact  bed  greater,  when  treating  septic  effluent. 

Example  of  a  Single  Contact  Bed — A  single  contact  bed 
is  shown,  in  perspective  in  Fig.  51.  The  tank  enclosing  a 
contact  bed  must  be  watertight,  and,  for  this  reason,  is  gen- 
erally made  of  masonry  or  concrete.  Sometimes,  however, 
puddled  clay  reservoirs  are  used  for  this  purpose,  while  in 
other  installations  the  bottom  is  of  clay  and  the  walls  of 
concrete.  In  many  respects  a  contact  bed  resembles  an 
intermittent  filter.  The  system  of  underdrains  is  the  same, 
with  the  exception  that  the  outlet  is  controlled  by  a  sluice 
gate,  a,  to  cut  off  the  flow  and  hold  the  sewage  in  contact 
for  the  full  period.  The  filling  material  for  a  contact  bed 


SEWAGE    PURIFICATION    AND    DISPOSAL  121 

is  much  coarser  than  the  sand  in  an  intermittent  filter  bed, 
but  is  of  about  the  same  depth.  Distributors  for  contact 
beds  are  similar  to  those  used  to  distribute  sewage  on  inter- 
mittent sand  filters,  although,  instead  of  wooden  or  concrete 
sluices,  trenches  may  be  made  of  cinders,  which  will  allow 
the  liquid  portion  of  the  influent  to  seep  through  and  at  the 
same  time  act  as  a  strainer  to  hold  back  the  coarser  par- 
ticles of  matter.  When  the  cinder  sluices  are  used  they  are 
shaped  roughly  like  wooden  sluices  and  likewise  have 
branches  to  conduct  sewage  to  various  parts  of  the  bed. 
The  surface  of  the  filling  material  is  not  provided  with 

r~  -Inlet. 


Fig.  51 

pavement  where  water  leaves  the  troughs,  however,  as  the 
material  of  the  contact  bed  is  too  coarse  to  be  easily  dis- 
turbed, and  even  if  it  were  easily  disturbed  it  is  not  vitally 
important  to  keep  the  surface  of  the  bed  level. 

Material  for  Contact  Beds — The  materials  suitable  for 
contact  beds  are  so  numerous  that  fortunately  some  suitable 
material  can  be  found  in  every  locality.  It  is  advisable 
when  selecting  a  material  to  choose  one  which  while  it 
might  not  be  the  best  so  far  as  efficiency  is  concerned,  will  at 
all  events  be  permanent  and  thus  keep  down  the  operating 
expenses  of  the  plant.  Coke  and  coal  seem  particularly 


122  SEWAGE    PURIFICATION    AND    DISPOSAL 

favorable  for  contact  beds,  but  when  cost  is  considered 
it  might  be  found  advisable  to  use  some  poorer  material 
that  can  be  had  at  less  cost,  depending  if  necessary  on 
double  contact  beds  to  produce  equal  results. 

The  presence  of  iron  in  a  contact  bed  seems  to  exert  a 
favorable  influence,  which  might  account  for  the  results 
obtained  by  use  of  broken  bricks  and  other  burnt  clay 
products.  Rough  materials,  as  a  rule,  give  better  results 
than  smooth  materials. 

It  would  seem  that  coke,  coal,  slag,  bricks,  gravel  and 
broken  stone  rank  in  efficiency  in  about  the  order  named. 
Broken  stone  filters  are  not  particularly  efficient.  How- 
ever, on  account  of  the  availability  and  the  permanency  of 
stone,  contact  beds  of  this  material  are  extensively  used. 
Burnt  clay  in  the  form  of  bricks,  broken  tile,  etc.,  give  very 
good  results,  but  unless  the  bricks  are  hard  and  the  tile  vit- 
rified, they  break  down  badly,  clogging  the  bed,  decreasing 
its  capacity  and  necessitating  cleaning. 

It  might  be  stated  that,  as  a  rule,  any  hard,  durable 
material  which  can  be  broken  or  crushed  into  particles  of 
the  required  size  may  successfully  be  used  as  a  filling  mate- 
rial for  contact  beds.  Various  kinds  of  stone,  such  as  trap- 
rock,  granite,  slate  and  limestone,  also  glass,  have  been 
used,  and  under  proper  conditions  give  satisfactory  results. 
Whatever  material  is  used,  however,  it  must  be  screened 
and  freed  from  dust  and  fine  particles  before  being 
deposited  in  the  contact  basin. 

Size  of  Material  for  Contact  Beds — The  size  of  material 
for  a  contact  bed  is  of  more  importance  than  the  kind  of 
material  used.  As  might  be  expected,  the  smaller  the 
material  of  a  contact  bed  the  greater  the  degree  of  purifi- 
cation attained.  The  reason  is,  that  the  finer  the  material 
of  a  contact  bed,  the  greater  is  the  active  surface  occupied 
by  the  micro-organisms,  and  with  which  the  organic  matter 
in  the  sewage  comes  in  contact.  Greater  purification 
would,  therefore,  follow  as  a  natural  sequence.  This 
higher  degree  of  purification,  however,  will  be  accompanied 
by  greater  loss  of  capacity,  particularly  if  the  fine  material 


SEWAGE    PURIFICATION    AND    DISPOSAL  123 

be  used  in  a  primary  contact  bed.  For  this  reason,  where 
.double  contact  beds  are  constructed,  the  primary  bed  is  filled 
with  a  coarse  material  of  from  about  y$  inch  to  i^  inches 
in  size,  and  the  secondary  beds  are  filled  with  material  from 
about  J8  inch  to  y$  inch  in  size.  The  material  in  the 
primary  contact  bed  is  supposed  to  be  large  enough  to 
admit  into  its  voids  the  particles  of  suspended  matter  car- 
ried by  the  sewage,  yet  fine  enough  to  prevent  their  being 
washed  out  onto  the  secondary  beds.  The  fine-grained 
secondary  bed,  on  the  other  hand,  will  admit  into  its  inter- 
stices any  suspended  matter  discharged  from  the  primary 
bed  and  will  produce  an  effluent  free  from  suspended 
matter. 

Depth  of  Material  for  Contact  Beds — Contact  beds  are 
usually  filled  to  a  depth  of  from  4  to  5  feet  with  the  contact 
material.  Less  depth  than  4  feet  could  be  used,  but  in  that 
case  the  area  covered  by  the  beds  would  have  to  be 
increased  to  make  up  for  the  decreased  capacity  caused  by 
the  shallowness  of  the  beds.  Greater  depths  than  5  feet 
are  not  advantageous  and  would  require  walls  of  too  great 
thickness  to  confine  the  head  of  water  within.  If  the  filling 
material  be  under  y%  inch  in  size,  the  depth  of  bed  should 
not  exceed  3  feet. 

Liquid  Capacity  of  Contact  Beds — Contact  beds,  when 
first  put  into  service,  have  a  liquid  capacity  of  about  50  per 
cent,  of  the  total  cubical  contents  of  the  bed.  The  original 
liquid  capacity,  however,  is  soon  reduced  to  about  33  per 
cent,  by  the  settling  together  of  the  material;  growth  of 
organisms ;  breaking  down  of  material ;  impaired  drainage 
and  insoluble  matter  entering  the  beds.  With  progressive 
use,  the  capacity  decreases  until  a  point  is  reached  where 
the  filtering  material  must  be  removed  from  the  beds  and 
cleaned  or  renewed.  This  is  one  of  the  most  objectionable 
and  costly  features  in  the  operation  of  a  contact  bed. 

The  original  capacity  of  contact  beds  can  be  restored 
to  a  considerable  extent  by  allowing  the  beds  to  rest 
empty  for  several  weeks.  This,  of  course,  will  affect 
only  the  organic  matter  and  growth,  and  cannot  affect  the 


124  SEWAGE    PURIFICATION    AND    DISPOSAL 

accumulated  mineral  matter.  In  practice  it  is  generally 
assumed  that  after  being  in  service  awhile,  the  liquid 
capacity  of  a  contact  bed  treating  septic  effluent  will  be  33 
per  cent,  of  its  cubical  capacity,  and  when  treating  crude 
sewage  25  per  cent,  of  its  cubical  capacity.  The  liquid 
capacity  of  a  contact  bed  is  determined  by  measuring  the 
effluent  drawn  off  when  a  bed  is  emptied. 

Operation  of  Contact  Beds — Contact  beds  are  operated 
in  a  series  of  cycles.  Usually  the  cycle  consists  of:  One 
hour  filling,  two  hours  resting  full  "in  contact,"  one 
hour  for  draining  and  four  hours  resting  empty — a  cycle 
of  eight  hours,  after  which  it  is  again  filled.  When 
carefully  managed  and  operated,  double  contact  beds 
can  receive  three  fillings  in  twenty-four  hours,  while 
for  single  contact  beds,  two  fillings  give  the  best  results. 
Two  fillings  a  day  give  better  results  than  one  filling. 
It  is  probable  that  one  filling  does  not  maintain  the 
bacteria  in  their  maximum  effectiveness.  The  distribu- 
tion of  fillings  at  regular  intervals  over  the  twenty-four 
hours  is  not  necessary  to  the  successful  working  of  a  plant. 
When  a  bed  is  first  started  the  purification  is  very  slight, 
but  the  bacteria  soon  multiply  enormously,  clinging  in 
colonies  to  the  surfaces  of  the  filling  material.  In  about 
two  or  three  weeks  the  bed  becomes  ripe,  after  which  it 
will  maintain  its  efficiency  indefinitely  if  properly  handled. 
A  sewage  which  is  slightly  alkaline,  the  normal  condition 
of  ordinary  domestic  sewage,  is  the  most  favorable  for 
bacterial  activity  in  a  contact  bed.  Very  alkaline  effluents, 
however,  such  as  those  produced  by  the  use  of  lime  in 
excessive  quantities,  are  very  liable  to  putrefy  instead  of 
being  purified  by  oxidizing  organisms.  That  is  probably 
why  chemical  treatment  with  lime  interferes  with  the 
process  in  a  contact  bed.  The  period  of  contact  is  an 
important  consideration  in  the  operation  of  a  contact  bed. 
If  sufficient  time  is  not  allowed,  the  purification  will  be 
incomplete,  while,  if  the  period  of  contact  be  carried  too 
far,  the  aeration  of  the  bed  and  consequent  recuperation 
are  unfavorably  affected.  Two  hours  have  been  found  to 


SEWAGE    PURIFICATION    AND    DISPOSAL  125 

be  about  the  best  period  of  contact;  the  effluent  is  then 
allowed  to  escape,  while  the  organic  matter  held  in  the  bed 
is  attacked  by  the  micro-organisms  under  the  most  favor- 
able conditions  of  dampness  and  air  supply. 

Capacity  of  Contact  Beds — The  rate  at  which  contact 
beds  can  be  operated  depends  on  the  strength  of  the 
applied  sewage  and  the  condition  in  which  it  is  delivered 
to  the  beds.  A  greater  quantity  of  weak  domestic  sewage, 
which  has  been  subjected  to  septic  action,  can  be  treated 
than  of  strong  sewage,  which  is  only  strained  or  subjected 
to  a  short  period  of  sedimentation.  The  number  of  fillings 
which  a  contact  bed  will  stand  also  has  a  marked  effect  on 
the  quantity  of  sewage  that  can  be  purified.  Usually  a 
contact  bed  can  treat  more  septic  effluent  than  it  can  crude 
sewage,  but  at  Hamburg  the  converse  was  true.  There 
the  contact  beds  could  handle  six  doses  of  crude  sewage  a 
day,  while  only  two  doses  of  septic  effluent  could  be 
applied,  a  third  dose  producing  a  dark,  malodorous  effluent. 

In  the  present  state  of  sewage  purification  practice  by 
the  contact  method,  three  fillings  each  day  is  about  the 
maximum  rate  for  primary  beds,  while  two  fillings  per  day 
will  give  the  best  average  results.  When  a  double  contact 
system  is  used,  the  secondary  beds  usually  occupy  about 
one-half  the  area  of  the  primary  beds  and  are  operated  at 
double  rates.  The  quantity  of  sewage  which  can  be  puri- 
fied per  acre  of  surface  under  average  conditions  varies 
from  about  .8  million  gallons  per  day  to  1.2  million  gallons 
per  day,  with  perhaps  a  mean  of  i  million  gallons  per  acre 
per  day,  which  may  safely  be  assumed  as  the  average 
capacity  of  a  contact  bed.  Experimental  contact  beds 
operated  in  Boston  for  the  study  of  the  best  treatment  for 
Boston  sewage  were  operated  most  satisfactorily  with  three 
fillings  a  day,  giving  a  single  contact  rate  for  6-foot  beds, 
with  fine  stone  filling,  of  1.2  million  gallons  per  acre  per 
day;  with  coarse  stone,  1.4  million  gallons  per  day;  1.8 
million  gallons  with  2-inch  coke  and  with  2-inch  brick; 
it  was  found,  however,  that  with  any  material  other  than 
the  ^-inch  stone  a  second  contact  would  be  necessary, 


12C> 


SEWAGE    PURIFICATION    AND    DISPOSAL 


reducing1  the  rate  on  the  double  system  as  a  whole  to 
between  .6  and  i  million  gallons.  Table  XII,  compiled  by 
Winslow  and  Phelps,*  shows  the  rates  which  have  recently 
been  obtained  in  actual  operation  or  in  experiment  on  a 
practical  scale. 

Efficiency  of  Contact  Beds — Single  contact  rarely  yields 
a  stable  effluent  without  the  beds  clogging  to  such  an 
extent  that  they  necessitate  the  removal  of  the  material 
several  times  a  year  for  cleaning. 

TABLE  XII — CONTACT  FILTER  RATES 

SINGLE    CONTACT 


Rate 

Place 

Depth 
Feet 

Million 

Gallons 
per  Acre 

per  Day 

Manchester     .         ..,"/,.         •         •         •• 

3.3 

.6 

Birmingham    .         .                  . 
Croydon           .         .         y         .    •     .         .         . 

4.5 
3.7 

.6 

.8 

Exeter     .        .        .        .        .                         .       •.; 

5 

1 

Sutton     .        .        H 

3.5 

1 

London    .         .         .         .... 

3 

1.2  - 

Leeds       .         .         .         .         . 

4.5 

1.4 

DOUBLE    CONTACT 

Burnley  .         .         ."        .         .       •."*'     .  "     . 
Leeds      .        ....        

3 
5.5 

.3 
.6 

Blackburn        .         

5.5 

.8 

Sheffield          .        .        /       

3.3 

.8 

Carlisle    .         .     -  

4 

1.1 

Sheffield           .         

3.3 

1.2 

Double  contact,  on  the  other  hand,  will  produce  a 
stable  effluent,  but  one  which  is  much  inferior  to  that  of  a 
sprinkling  filter  or  intermittent  sand  filter.  On  an  average, 
primary  contact  will  remove  50  per  cent,  of  the  dissolved 
impurity  in  the  applied  sewage,  and  secondary  contact  will 
remove  about  50  per  cent,  of  the  organic  matter  in  the 
effluent  from  the  primary  contact  bed.  Ordinarily,  then, 
it  can  be  assumed  that  from  70  to  80  per  cent,  of  the 


Investigations  on  the  Purification  of  Boston  Sewage. 


SEWAGE    PURIFICATION    AND    DISPOSAL  127 

organic  matter  in  the  sewage  applied  to  double  contact 
beds  will  be  disposed  of.  The  effluents  from  contact  beds 
are  not  always  clear,  although  they  may  be.  They  are 
generally  non-putrescible,  but,  bacteriologically,  they  are 
not  good  enough  to  discharge  into  streams  which  are  used 
for  public  water  supply,  nor  into  tidal  estuaries  where 
there  are  oyster  beds. 

Effect  of  Temperature  on  Contact  Beds — It  is  quite 
probable  that  contact  beds  can  be  successfully  operated 
during  cold  weather  in  any  locality  where  intermittent 
sand  filters  can  be  operated.  The  whole  process  occupies 
only  a  few  hours  in  each  bed,  and  there  is  not  sufficient 
time  for  such  a  bulk  of  sewage  to  cool  down  perceptibly. 
However,  the  purification  effected  during  the  winter 
months  will,  no  doubt,  be  considerably  less  than  during  a 
like  period  of  warm  weather,  for  the  greater  number  of 
micro-organisms  active  in  purifying  sewage  can  thrive 
only  when  the  temperature  ranges  between  50  and  100 
degrees  Fahrenheit,  and,  as  in  winter  weather  the  temper- 
ature of  the  sewage  will  fall  to  from  35  to  40  degrees  Fah- 
renheit, only  a  comparatively  small  number  of  bacteria 
will  be  active. 

Double  Contact  Beds — Double  contact  beds  are  built 
with  the  secondary  beds  at  a  lower  elevation  than  the 
primary  beds,  so  that  the  effluent  from  the  primary  beds 
can  discharge  by  gravity  onto  the  secondary  beds.  Gener- 
ally the  application  of  sewage  to  contact  beds  is  on  the 
surface,  although  there  is  a  tendency  at  the  present  time 
to  fill  the  beds  from  below,  and  allow  the  liquid  to  rise  in 
the  filtering  material,  expelling  the  air  from  the  voids. 
Until  time  and  experience  demonstrate  the  wisdom  of  that 
manner  of  filling  contact  beds,  however,  the  better  practice 
will  be  to  apply  the  sewage  or  influent  to  the  surface  of 
the  bed. 

Automatic  Apparatus  for  Contact  Beds — Usually  con- 
tact beds  are  operated  by  hand  labor,  as  it  is  necessary  to 
have  an  attendant  at  the  plant  who  can  operate  the  sluice 
gates.  In  some  small  plants,  however,  airlock  apparatus 


128  SEWAGE    PURIFICATION    AND    DISPOSAL 

is  used  to  operate  the  gates,  thus  insuring  a  uniform  period 
of  fill,  contact  and  draw. 


SEWAGE  IRRIGATION 

Principles  of  Sewage  Irrigation — In  sewage  irrigation, 
or  sewage  farming,  as  it  is  sometimes  called,  the  crude 
sewage,  after  having  been  passed  through  a  screen  cham- 
ber and  detritus  tank  to  remove  the  coarser  solids,  is 
applied  to  the  surface  of  ordinary  agricultural  land,  to  pro- 
vide moisture  for  the  growing  vegetation,  and  at  the  same 
time  to  enrich  the  soil  with  the  plant  food  which  is  carried 
both  in  suspension  and  solution  by  the  sewage.  When 
sewage  irrigation  is  resorted  to,  purification  of  the  sewage 
is  usually  secondary  to  irrigation,  consequently  larger  areas 
of  land  are  required  than  when  purification  of  the  sewage 
is  the  chief  consideration.  The  large  area  of  land  required 
for  sewage  irrigation,  however,  insures  a  thorough  purifi- 
cation of  the  applied  sewage,  providing  the  irrigation  fields 
have  been  properly  prepared  for  the  purpose.  The  prin- 
ciples which  underlie  the  practice  of  ordinary  irrigation  are 
what  must  be  followed  in  sewage  farming.  There  is  no 
special  treatment  of  the  sewage  required  outside  of  strain- 
ing. The  method  of  preparing  the  soil,  underdraining, 
flooding  and  cropping  are  the  same,  whether  sewage  or 
fresh  water  is  applied.  Raising  of  crops  is  the  chief  con- 
sideration in  sewage  irrigation,  as  it  is  in  ordinary  irriga- 
tion, and  the  fields,  if  prepared  for  that  pur-pose,  will 
completely  purify  the  sewage  which  is  applied,  provided 
judgment  is  shown  in  the  dosing  of  fields  not  to  apply  too 
much  sewage  at  one  time  or  at  too  frequent  intervals. 
When  the  amount  of  sewage  applied  to  a  given  area  is  not 
excessive,  the  organic  solids  are  gradually  dissolved  with 
the  formation  of  soluble  products  suitable  for  the  food  of 
higher  plants,  and  the  liquid  seeps  away  to  join  the  ground 
water,  or  is  carried  off  in  underdrains,  as  the  case  may  be. 
If,  however,  the  fields  are  overdosed,  they  become  sewage 
sick;  the  surface  clogs,  pools  are  formed,  putrefaction 


SEWAGE    PURIFICATION    AND    DISPOSAL  129 

begins  and  a  stench  arises.     Only  a  complete  rest  will  then 
restore  the  fields  to  their  normal  condition. 

Any  soil  that  is  suitable  for  agricultural  purposes  will 
be  found  suitable  for  sewage  farming  or  irrigation. 


SOILS 

Composition  of  Soils — All  soils  suitable  for  farming  are 
made  up  of  varying  proportions  of  sand,  humus,  silt  and 
clay.  No  one  of  these  ingredients  alone  makes  a  suitable 
soil,  nor  is  it  possible  to  find  any  one  of  these  ingredients 
which  is  not  mixed  to  a  greater  or  less  extent  with  the 
others.  Loams,  the  most  valuable  of  farming  soils,  are  a 
mixture  of  the  four  ingredients.  This  mixture  of  different 
materials  is  beneficial  not  only  in  improving  the  texture  of 
the  soil,  but  also  in  providing  suitable  plant  food,  without 
which  a  soil  would  be  barren.  All  of  the  soils  from  which 
crops  are  raised  contain  at  least  seven  elements  required 
for  plant  food ;  these  are  nitrogen,  potassium,  phosphorus, 
calcium,  iron,  magnesium  and  sulphur;  without  these  no 
plants  grow.  Nitrogen,  potassium,  phosphorus  and  calcium 
are  much  needed  by  plants,  and  so  the  soil  is  liable  to 
become  exhausted  of  them  if  not  fertilized.  The  iron, 
magnesium  and  sulphur  are  usually  so  abundant  as  to  be 
practically  inexhaustible. 

Classification  of  Soils — The  relative  amounts  of  sand, 
clay  and  humus  in  a  soil  influence  its  texture,  and  serve  as 
a  broad  classification  for  agricultural  soils  into  sandy  soils, 
clayey  soils  and  humus  soils.  Besides  these  there  are  the 
loams,  which  are  known  as  sandy  loams  and  clayey  loams. 

Soils  which  contain  80  per  cent,  of  sand  and  less  than 
10  per  cent,  of  clay  are  known  as  sandy  soils.  They  usually 
are  leachy — especially  if  the  sand  grains  are  large — and  are 
poor  in  plant  food.  Fine-grained  sandy  soils,  as  a  rule,  are 
better  than  coarse  sandy  soils. 

Soils  which  contain  from  60  to  70  per  cent,  of  sand  and 
20  to  30  per  cent,  of  humus  are  generally  considered  sandy 
loams,  while  soils  which  contain  70  to  80  per  cent,  of  sand 


130  SEWAGE    PURIFICATION    AND    DISPOSAL 

and  10  to  20  per  cent,  of  humus  are  considered  light  sandy 
loams.  Usually,  soils  with  an  open,  porous  texture,  like 
sand  loams,  are  considered  light,  while  soils  which  are  of 
close  texture,  like  clay,  are  said  to  be  heavy. 

Soils  which  contain  60  per  cent,  or  more  of  clay  or  silt 
are  commonly  called  clay  soils.  When  the  percentage  of 
clay  or  silt  reaches  80  per  cent,  or  90  per  cent,  the  soil  is 
worthless  for  farming.  Clay  loams  are  similar  to  clay  soils, 
from  which  they  differ  chiefly  in  the  less  amount  of  clay 
they  contain.  A  soil  containing  from  30  to  40  per  cent,  of 
clay  is  said  to  be  a  clay  loam,  while  a  soil  which  contains 
from  40  to  50  per  cent,  of  clay  is  classed  as  a  heavy  clay 
loam.  A  clay  loam  usually  contains  from  25  to  35  per 
cent,  of  sand  and  a  heavy  clay  loam  contains  from  10  to 
25  per  cent,  of  sand. 

Loams,  which  are  the  most  valuable  farming  soils, 
contain  from  40  to  60  per  cent,  of  sand  and  15  to  25  per 
cent,  of  clay. 

Voids  in  Soils — About  50  per  cent,  of  the  volume  of 
ordinary  soils  is  space,  which  is  filled  with  air  and  water. 
Air  is  as  necessary  as  water  to  agricultural  soil.  Nitrifying 
bacteria,  which  prepare  organic  matter  in  the  humus,  ferti- 
lizer or  sewage  for  plant  food,  can  perform  their  functions 
only  in  the  presence  of  air  or  oxygen ;  therefore,  a  moist, 
well  drained  soil,  of  open  texture,  which  permits  a  free  cir- 
culation of  ground  air,  is  the  best  for  bacterial  activity,  as 
well  as  for  the  growth  of  farm  crops.  The  texture  of  soils 
varies  much  with  the  composition  or  the  relative  amounts 
of  each  material  the  soils  contain.  For  instance,  clay  occu- 
pies about  65  per  cent,  of  space,  leaving  only  35  per  cent, 
of  voids,  while  a  sandy  truck  soil  contains  37  per  cent,  of 
space,  leaving  63  per  cent,  of  voids.  Soils  of  other  textures 
vary  all  the  way  between  these  extremes. 

The  comparative  fineness  of  different  materials  from 
which  soil  is  composed  can  be  seen  in  Table  XI. 

Soil  Moisture — A  good  farm  soil  often  holds  more  than 
one-half  its  weight  of  film  water,  after  the  free  water  has 
passed  off.  As  a  result  of  a  force  known  as  surface  tension, 


SEWAGE    PURIFICATION    AND    DISPOSAL  131 

each  particle  of  soil  holds  a  film  of  water  over  its  entire 
surface,  and  no  matter  how  well  drained  a  soil  may  be,  or 
how  dry  it  might  appear  on  the  surface,  this  film  is  always 
to  be  found  on  the  surface  of  the  soil  particles.  Plants1  de- 
rive their  .nourishment  from  this  film  water,  and  Hot  from 
the  ground  water  or  free  water  which  sometimes  fills  the 
pores  or  voids  of  the  soil.  The  moisture  necessary  to  sup- 
ply the  surface  films  moves  through  the  soil  independent  of 
the  force  of  gravity,  impelled  by  the  force  of  capillary  attrac- 
tion or  surface  tension.  Naturally,  the  precentage  of  mois- 
ture differs  at  different  depths  of  the  soil  and  at  different 
points  in  the  field.  For  instance,  near  the  surface,  where 
evaporation  is  rapid,  capillarity  cannot  supply  moisture  fast 
enough  to  maintain  an  even  distribution.  Again,  particles 
which  are  in  contact  with  the  fine  hair-like  roots  of  plants 
yield  their  film  moisture  readily  to  the  plants.  These  parti- 
cles, however,  are  in  contact  with  other  grains,  which  are  not 
exposed  to  the  capillary  attraction  of  roots,  and  these  latter" 
grains  yield  part  of  their  moisture  to  the  drier  particles', 
drawing  in  turn  for  a  supply  to  replenish  their  films  fforrt 
the  more  moist  particles  with  which  they  are  in  contact. 
In  this  manner  soil  moisture  moves  through  the  soil,  up  or 
down,  crosswise  or  horizontally,  independent  of  the  laws  of 
gravity  and  of  the  flow  of  ground  water.  Any  moisture  in 
excess  of  the  film  water  is  known  as  ground  water.  This 
excess  water  is  not  only  of  no  value  in  soil,  but  is  actually 
injurious,  as  it  drowns  vegetation. 

To  be  valuable  for  raising  crops,  soil  must  be  thor- 
oughly underdrained,  either  naturally  or  artificially,  to  lower 
the  ground  water,  so  the  soil  and  subsoil,  as  far  as  roots 
penetrate,  will  have  only  the  film  water  to  draw  upon,  and 
a  plentiful  supply  of  oxygen  or  air  to  prepare  the  plant  food 
in  the  film  water  for  consumption. 

A  coarse  sand  holds  about  12  to  15  per  cent,  by  weight 
of  film  water;  a  sandy  loam  from  20  to  30  per  cent. ;  a  clay 
loam  from  30  to  40  per  cent.,  and  a  heavy  clay  or  a  soil  very 
rich  in  humus  may  hold  40  to  50  per  cent,  of  film  water. 
That  is  to  say,  a  mellow  loam,  with  a  retentive  subsoil, 


132  SEWAGE    PURIFICATION    AND    DISPOSAL 

might  hold  from  5  to  6  inches  of  water  in  a  foot  of  the  top 
soil. 

It  will  be  noticed  that  the  heavier  a  soil  and  the  finer 
the  particles  the  more  moisture  it  will  hold.  That  would 
naturally  follow  from  the  fact  that  if  a  i-inch  cube  of  stone 
be  granulated,  the  grains  will  present  more  surface  than 
the  original  cube,  and  the  finer  the  stone  is  granulated  the 
more  surface  it  will  present.  While  fine  clayey  soils  hold 
more  film  moisture  than  do  sandy  soils,  they  cling  to  it  more 
tenaciously  and  give  up  to  the  plants  a  much  less  percen- 
tage than  do  sandy  soils.  The  liquid  which  enters  the  pores 
of  a  soil  displaces  the  liquid  or  air  which  was  previously 
present.  The  air  is  forced  upward  into  the  atmosphere  and 
the  water  is  forced  downward  to  the  underdrains  or  to  the 
water-table.  In  order  that  sewage  when  applied  to  agri- 
culttiral  land  will  not  pass  directly  through  to  the  under- 
drain,  the  quantity  must  not  be  greater  than  can  be  taken 
up  by  the  pores  of  the  soil. 

Owing  to  the  greater  capillary  attraction  of  the  small 
grains  of  clay  over  that  of  sand,  the  former  will  raise  water 
from  a  greater  depth  than  will  sand.  This  strong  capil- 
larity tends  to  keep  the  soil  moist  and  on  this  account  the 
underdrains  in  artificial  drainage  should  be  nearer  the  sur- 
face when  draining  heavy  lands  than  when  draining  light 
sandy  soils. 

Temperature  of  Soils — The  temperature  of  soils  has 
much  to  do  with  their  value  for  cropping,  and  the  tem- 
perature depends  to  a  great  extent  upon  their  texture, 
composition,  color  and  exposure.  Sandy  soils  are  warm 
because  the  large  quartz  grains  hold  heat  well,  and  the 
coarser  the  sand  the  warmer  it  gets  and  the  better  it  holds 
the  heat.  Clay,  on  the  other  hand,  warms  much  faster 
than  sand,  because  the  particles  lie  closer  together  so  that 
heat  can  pass  more  readily  from  one  particle  to  another. 
For  the  same  reason  clayey  soils  lose  more  heat  by  radiation 
than  do  sandy  soils,  and,  as  they  hold  more  water  than 
sandy  soils,  they  lose  additional  heat  by  reason  of  the 
greater  amount  of  evaporation  from  the  surface.  So  it  is 


SEWAGE    PURIFICATION    AND    DISPOSAL  133 

that  fine-grained  soils  are  colder  than  coarse-grained  soils, 
although  they  absorb  more  heat ;  and  sandy  soils  are  con- 
sidered warm,  while  clay  soils  are  said  to  be  cold. 

Color  of  soils  also  has  a  thermal  effect.  Dark-colored 
soils  absorb  more  of  the  heat  rays  of  the  sun  than  do  light- 
colored  soils  of  equal  texture  and  composition;  con- 
sequently, in  northern  latitudes,  dark-colored  soils  are 
preferable  to  light-colored  soils  for  irrigation  or  sewage 
farms.  Fields  on  the  southern  slope  of  a  hill,  or  on  the 
level  on  the  southern  side  of  a  range  of  hills,  where  they 
are  protected  from  the  wind  and  exposed  to  the  sun,  will 
be  much  warmer  than  fields  with  northern  or  western 
exposure. 

Drainage  of  Soils — All  soils  used  for  raising  crops  should 
have  good  drainage,  and  unless  the  surface  soil  is  underlaid 
by  a  porous  subsoil  of  sufficient  depth  to  ensure  the  removal 
of  all  water  seeping  through  the  upper  layers,  the  soil 
should  be  underdrained  with  tile  drains,  if  the  fields  are  to 
be  used  for  sewage  irrigation.  Underdrainage  lowers  the 
water-table,  allows  all  surplus  water  to  flow  off  so  that  film 
water,  in  the  presence  of  air,  will  be  available  for  the  plants 
to  feed  upon.  When  the  water-table  is  lowered,  the  roots 
of  growing  plants  shoot  downward,  following  the  receding 
water  level,  and  thus  so  much  more  available  soil  is  added 
to  the  field.  For  instance,  if  the  ground  water  in  a  field  is 
within  14  inches  of  the  surface,  there  are  only  14  inches  of 
soil  for  the  roots  to  occupy  and  the  available  food  and  air 
are  reduced  almost  to  limits  of  barrenness.  If,  however, 
the  field  be  underdrained  so  as  to  lower  the  water  table  to 
4  feet  below  the  surface,  about  three  and  one-half  times 
the  original  growing  soil  will  thus  be  made  available. 
Underdrains  not  only  carry  off  the  surplus  water  from  a 
field,  but  they  also  promote  aeration  of  the  soil. 

Preparing  Soils  for  Irrigation — Suitable  soils  of  a  light 
sandy  texture,  underlaid  with  porous  strata  of  material  to 
give  good  natural  drainage,  are  not  always  available  for 
sewage  irrigation,  and  areas  of  land,  such  as  are  obtainable 
must  be  put  in  shape  for  the  purpose.  A  sandy  loam  is  the 


134  SEWAGE    PURIFICATION    AND    DISPOSAL 

most  desirable  soil  for  irrigation.  The  grains  being  coarse, 
permit  the  ready  passage  of  water  through  them  after  the 
capillary  spaces  have  been  filled,  thus  permitting  the  treat- 
ment of  a  large  quantity  of  sewage.  The  soil  is  sufficiently 
retentive  to  store  a  supply  of  film  water  for  plant  food; 
capillary  action  is  strong  enough  to  raise  water  from  the 
underground  reservoir  to  the  plant  roots.  Heat  is  given  off 
very  slowly,  so  the  soils  are  warm;  fields  can  be  worked 
within  a  short  time  after  a  rainfall  or  after  sewage  has  been 
applied ;  crops  can  be  sowed  at  least  two  weeks  earlier  than 
in  heavy  soils,  and  the  open  texture  of  the  soil  permits  of 
thorough  aeration.  There  are  two  conditions,  however, 
under  which  such  soils  are  not  suitable  for  irrigation ;  these 
are  when  the  water-table  is  too  high  or  when  it  is  too  low, 
and  the  remedy  for  either  condition  is  underdrainage. 
When  the  water-table  is  too  high,  the  excess  of  moisture 
reduces  the  temperature  of  the  soil,  excludes  the  air  and 
dilutes  the  plant  food,  thereby  retarding  or  stopping  entirely 
the  growth  of  the  plants.  Further,  it  submerges  and  ren- 
ders inaccessible  to  the  roots  of  plants  great  quantities  of 
plant  food  stored  in  the  subsoil,  which  can  be  reclaimed  by 
lowering  the  water  level.  In  arid  climates  when  the  sub- 
soils contain  considerable  quantities  of  soluble  alkali  salts, 
such  as  sodium  chlorid,  sodium  sulphate  and  sodium  car- 
bonate, and  the  water-table  is  very  low,  sometimes  40  to 
60  feet  below  the  surface,  the  salts  become  dissolved  from 
the  soils,  and,  in  solution,  are  carried  by  capillary  attrac- 
tion toward  the  surface.  If  evaporation  in  such  localities 
be  rapid,  the  alkali  contained  in  the  water  will  be  deposited 
in  solid  form  near  the  surface,  and  the  land  will  deteriorate 
for  cropping. 

Clayey  soils  are  the  exact  antithesis  of  sandy  soils,  both 
in  texture  and  in  agricultural  value.  The  very  small  spaces 
between  the  exceedingly  fine  grains  of  a  clayey  soil  admit 
air  and  water  very  slowly,  but  hold  the  water  tenaciously, 
so  that  when  a  clayey  soil  becomes  thoroughly  wet  it  is 
sticky.  When  the  soil  becomes  dry  it  cracks,  thus  opening 
wide  and  numerous  crevices,  through  which  sewage  can 


SEWAGE    PURIFICATION    AND    DISPOSAL  135 

pass  to  the  tmderdrains.  If  turned  over  by  a  plow  while 
in  a  wet  condition  the  soil  bakes  and  becomes  cloddy. 
When  clayey  soils  are  the  only  soils  which  are  available  for 
sewage  irrigation,  they  should  be  treated  to  remedy  the 
chief  defect,  which  is  heaviness.  Underdrains  will  remove 
the  surplus  water,  and  promote  aeration  and  warmth. 
Judicious  cultivation  will  also  do  much  to  improve  the 
texture  of  the  soil. 

The  fine  particles  of  clay  can  be  separated  from  one 
another  by  mixing  with  humus,  stable  manure,  green 
manure  or  sand,  and  plowing  under  deep.  If  sand  or 
manure  are  not  available,  coal  ashes  mixed  with  the  clay 
will  give  excellent  results. 

Systems  of  Underdrains — Underdrainage  systems,  for 
sewage  irrigation  fields,  are  similar  in  principles  of  con- 
struction to  those  for  ordinary  farm  drainage.  The  object 
is  to  provide  a  system  of  underground  channels  large 
enough  to  carry  off  the  maximum  quantity  of  water  dis- 
charged into  them  and  to  have  the  branch  drains  so  spaced 
that  the  ground  water  between  the  drains  will  rise  but 
slightly  above  the  level  of  the  tiles.  The  most  economical 
system  for  thorough  underdrainage  is  to  have  the  branch 
drains  parallel  to  one  another,  and  discharge  into  a  main 
intercepting  line.  On  sloping  ground,  the  branches  may 
be  laid  up  and  down  the  slope,  diagonally  across  the  slope, 
or,  where  seepage  of  water  moves  laterally  down  the  slope 
from  above,  an  intercepting  line  may  be  extended  along 
the  upper  edge  of  the  slope  to  cut  off  the  seepage. 

The  plan  used  in  draining  a  tract  of  480  acres  of  open 
black  soil  with  clayey  subsoil  in  Iroquois  County,  111.,  is 
shown  in  Fig.  52.* 

This  land  was  generally  level  but,  before  drainage,  was 
dotted  with  ponds  which  contained  water  during  six  months 
of  the  year.  The  grades  at  which  the  drains  were  laid  were 
in  some  cases  i  inch  to  100  feet,  and  in  others  as  high  as  2 
inches  in  100  feet.  Each  line  on  the  plan  is  designated  by 
a  name  or  number  to  distinguish  it  from  the  others,  and  its 

*  From  Farmers'  Bulletin,  Drainage  of  Farm  Lands,  by  C.  G.  Elliott. 


136 


SEWAGE    PURIFICATION    AND    DISPOSAL 


length  and  size  as  well  as  its  junction  with  other  lines  is 
indicated  by  the  number  of  feet  or  the  station  number  from 

_4L  **  lf 


>SCALE:  or  FEZI 


200       400       600        800        IOOO 


Fig.  52 


the  outlet  point  in  each  case.     Such  a  map  is  valuable  for 
the  data  it  contains.    When  installing  the  pipes,  everything 


SEWAGE    PURIFICATION    AND    DISPOSAL 


137 


is  done  according  to  system,  previously  studied  out, 
while  in  case  of  a  stoppage  after  the  fields  are  put 
in  service,  the  exact  location  of  the  drains  can 
be  determined  without  unnecessary  digging. 
*v  Another  plan,*  Fig.  53,  shows  the  drain- 

age of  128  acres  of  clay  land  in  Jefferson 
County,  Kentucky.     By  comparing  this 
map  with   the  one  shown 
in  Fig.  52,  it  will  be  noticed 
that  there  is  a  considera- 
ble difference  in  the  sys- 
tems, necessitated  by  the 
difference   in   the   text- 
ure   and   composition 
of  the  soils.    In  the 
\        Kentucky  plan, 
which  was  de- 
signed for 


Fig.  53 


*  From  Farmers'  Bulletin,  Drainage  of  Farm  Lands,  by  C.  G.  Elliott. 


138  SEWAGE    PURIFICATION    AND    DISPOSAL 

clay  soil,  the  drains  are  laid  systematically  50,  60  and  70 
feet  apart,  while  in  the  Illinois  plan,  where  the  soil  pos- 
sessed better  drainage  properties,  the  system  was  designed 
with  special  reference  to  the  natural  drainage  of  the  land 
and  the  particular  requirements  of  unusually  wet  spots 
where  pools  of  water  stood  for  half  the  year. 

For  sewage  irrigation  work,  when  the  soil  is  of  approxi- 
mately uniform  texture,  the  plan  of  spacing  the  drains  at 
uniform  distances  would  probably  be  preferable. 

Aeration  of  the  soil  is  an  important  function  of  under- 
drains  which  is  particularly  important  and  of  very  great 
benefit  to  close  soils.  To  promote  aeration  surface  vents 
are  sometimes  connected  to  the  drains  to  induce  a  more 
rapid  circulation  of  air  through  the  drains  and  soil. 

Size  of  Underdrains  for  Irrigation  Fields — The  capacity 
of  drains  of  different  sizes  and  laid  at  various  grades  can  be 
quite  accurately  calculated,  but  the  quantity  of  water  to  be 
removed  by  a  system  of  underdrains  is  a  more  difficult 
problem  to  solve.  It  may  be  stated,  however,  as  a  rule 
applicable  to  all  underdrain  systems  for  irrigation  fields, 
that  they  should  have  sufficient  capacity  to  carry  off  within 
twenty-four  hours  the  excess  water  of  the  heaviest  rains 
that  fall.  If  they  are  made  sufficiently  large  to  care  for 
the  rainfall  they  will  be  large  enough  to  carry  off  the  small 
amount  of  effluent  which  seeps  through  to  the  underdrains. 
The  soil  is  a  great  reservoir  and  will  hold  from  25  to  40  per 
cent,  of  its  volume  of  water.  In  addition  to  this,  evapora- 
tion *  takes  place  rapidly  from  the  surface  of  soil  and  leaves 
of  vegetation,  reaching  even  in  moist  climates  the  high 
volume  of  70  per  cent,  of  the  rainfall.  It  would  seem, 
therefore,  that  an  underdrainage  system,  proportioned 
to  care  for  the  excess  rain  water,  would  be  of  sufficient 
size  under  any  combination  of  conditions  to  care  for 
sewage  effluent.  In  practice,  it  may  be  assumed  that  one 
inch  in  depth  of  water  must  be  removed  in  twenty-four 
hours  from  the  entire  irrigation  field,  and  the  underdrainage 


*  Each  square  foot  of  ordinary  farm  soil  loses  about  1.3  pounds  of  water  daily 
by  evaporation  from  the  surface. 


SEWAGE    PURIFICATION    AND    DISPOSAL  139 

system  should  be  proportioned  to  take  care  of  that  flow. 
If,  however,  the  soil  to  be  drained  is  a  retentive  one  and  in 
a  locality  not  subject  to  heavy  rains,  allowance  can  be  made 
to  remove  only  %  inch  of  rainfall.  Three-inch  tiles  are  the 
smallest  that  should  be  used.  In  Table  XIII  will  be  found 
the  areas  from  which  ^  incn  °f  water  will  be  removed  in 
twenty-four  hours  by  tile  drains  of  various  diameters  and 
different  lengths  when  laid  with  different  grades.  Should 
the  conditions  be  such  as  to  require  the  removal  of  greater 
depths  of  water  within  twenty-four  hours,  a  proportionate 
reduction  should  be  made  from  the  number  of  acres  a  pipe 
of  certain  size  will  drain.  For  instance,  when  laid  at  a 
grade  of  ^  inch  in  100  feet,  a  5-inch  pipe  1,000  feet  long 
will  drain  17.3  acres  of  land  of  %  inch  in  depth  of  water, 
and  4T3g-  acres  of  land  of  i  inch  depth  of  water. 

The  size  of  underdrain  pipe  is  increased  as  the  system 
nears  the  outlet  and  receives  the  drainage  of  the  larger 
area. 

Depth  of  Irrigation  Underdrains — The  most  advantage- 
ous depth  for  tile  underdrains  depends  much  on  the  charac- 
teristics of  the  soil  and  the  distance  apart  of  the  drains.  If 
shallow  drains  are  laid  wide  apart  in  clayey  soil,  it  is  pos- 
sible that  midway  between  the  drains  the  ground  water 
will  stand  near  the  surface.  In  that  case  the  obvious 
remedy  would  be  to  lay  another  drain  midway  between  the 
lines  already  installed.  The  same  result  could  be  obtained 
by  laying  drains  in  clayey  soils  deeper,  but  4  feet  is  the 
maximum  depth  that  the  drains  should  be  laid  in  such  soils, 
and  3  feet  is  perhaps  a  fair  average.  As  a  rule,  drains 
should  be  placed  as  deep  as  they  will  readily  receive  water, 
no  deeper.  In  sandy  soils  this  depth  ranges  from  4  to  6 
feet,  with  a  fair  average  of  5  feet. 

Distance  Between  Irrigation  Underdrains — To  secure 
efficient  drainage,  the  branch  lines  of  an  underdrainage 
system  should  be  placed  sufficiently  close  together  so  there 
will  be  no  appreciable  rise  in  the  level  of  the  ground  water 
between  any  two  drains.  The  distance  naturally  depends 
on  the  composition  of  the  soil,  water  traveling  more  freely 


140 


SEWAGE    PURIFICATION   AND    DISPOSAL 


TABLE  XIII* — SIZES  AND  CAPACITIES  OF  DRAIN  PIPES 


Diameter  of  tile  in 
inches 

Grade  per  100  feet  in  decimals  of  a  foot  (with  approxima  e  equivalents  in  inches) 

0.04(J-in.) 

0.05  (|-in.) 

0.08(l-in.)     |     fl-lOOA-in.) 

0.12(li-in.)     j     0.16  (2-in.) 

Length  of  drain  in  feet. 

1,000 

2,000  |   1.000 

2,000 

1,000 

2,000 

1,000 

2,000 

1,000 

2,000  |    1,000 

2,000 

Acres  of  land  drained. 

5  

g 

17.3 
27.3 
39.9 
55.7 
74.7 
96.9 
152.2 
222.8 
310.2 
414.4 
537.6 

13.5 

21.4 
31.4 
43.7 
58.8 
76.3 
119.9 
175.9 
245.0 
328.7 
426.9 

17.7 
28.0 
41.1 
57.3 
76.5 
99.5 
156.1 
228.7 
317.8 
424.9 
551.6 

14.0 
22.2 
32.7 
45.6 
61.2 
79.5 
124.9 
183.7 
255.9 
342.5 
444.9 

19.1 
29.9 
44.1 
61.4 
82.2 
106.7 
167.7 
245.3' 
341.4 
456.4 
591.5 

15.7 
24.8 
36.4 
50.7 
68.1 
88.5 
139.3 
204.3 
284.6 
381.3 
495.8 

19.8 
31.2 
45.9 
64.0 
85.6 
111.2 
174  8 
256.1 
355.4 
475.7 
616.4 

16.7 
26.4 
38.7 
53.9 
72.3 
94.0 
147.9 
217.4 
302.5 
405.5 
526.7 

20.6 
32.5 
47.7 
66.5 
89.1 
115.6 
181.7 
265.8 
369.5 
494.4 
640.4 

17:6 
27.8 
40.8 
57.0 
76.3 
99.2 
156.2 
229.7 
319.7 
428.1 
556.6 

22.1 

34.8 
51.1 
71.2 
95.3 
123.9 
194.6 
284.9 
396.3 
529.1 
686.3 

19.4 
30.5 

44.  S 
62.6 
83.8 
108.9 
171.6 
251.7 
350.4 
470.1 
610.5 

7  
8  

10  
12  

14  
16  

18 

20  

Diameter  of  tile  in 
inches 

Grade  per  100  feet  in  decimals  of  a  foot  (with  approximate  equivalents  in  inches). 

0.20(2|-in.)     |     0.25  (3  in.) 

0.30(3i-in.) 

0.40(4|-in.)     |     0.50  (6-in.)     |     0.75  (9-in.) 

Length  of  drain  in  feet. 

1,000  |   2,000 

1,000  |  2,000  |   1,000 

2,000  |    1,000 

2,000 

1,000 

2,000   |   1,000 

2,000 

Acres  of  land  drained. 

5 

23.5 
37.0 
54.3 
75.6 
101.4 
131.6 
206.8 
302.5 
420.6 
562.2 
729.2 

20.9 
33.0 
48.5 
67.7 
90.7 
117.9 
185.6 
272.2 
379.1 
508.1 
060.3 

25.1 

39.6 
58.0 
80.9 
108.4 
140.6 
221.1 
323.5 
449.9 
601.8 
780.0 

22.7 
35.9 
52.8 
73.6 
98.6 
128.1 
201.8 
296.1 
412.2 
552.5 
718.2 

26.7 
42.0 
61.6 
85.8 
114.9 
149.3 
234.5 
343.5 
477.4 
638.1 
826.9 

24.5 

11:? 

79.0 
106.0 
137.6 
216.9 
318.1 
442.9 
593.7 
771.1 

29.5 
46.4 
68.2 
95.0 
127.0 
165.2 
259.2 
379.7 
527.8 
705.2 
914.7 

27.5 
43.5 
63.8 
89.1 
119.4 
155.3 
244.1 
358.2 
498.4 
668.0 
867.8 

32.0 
50.5 
74.0 
103.3 
138.1 
179.2 
281.8 
412.9 
573.7 
767.4 
994.5 

30.3 
47.8 
70.1 
98.0 
131.3 
170.5 
268.6 
393.9 
548.8 
735.1 
954.6 

37.7 
59.4 
87.1 
121.4 
162.6 
211.1 
331.8 
485.8 
675.2 
902.3 
1.170.1 

J?:S 

84.1 
117.3 
157.1 
204.4 
321.7 
472.1 
657.3 
880.5 
1.144.1 

(i  
7  

g 

S::::::::;::::::::x::: 

10 

12 

14  

16 

18  
20 

*This  table  was  computed  by  C.  G.  Elliott  from  the  formulae  for  determining 
the  size  for  tile  drains  given  in  Elliott's  Engineering  for  Land  Drainage,  which  are  : 


.0105 

Where  #=velocity  of  flow  in  feet  per  second. 
a  =  sectional  area  of  tile  in  square  feet. 
df=diameter  of  tile  in  feet. 
_/=total  fall  in  length  of  drain. 
/=depth  of  drain  in  feet  at  upper  end. 
k= total  length  of  drain  in  feet. 
(9=discharge  of  drain  in  cubic  feet  per  second. 
^=acres  drained. 

Constant  0.0105=quantity  of  water  to  be  removed  from  1  acre  in  1  second 
of  time. 

Computations  are  made  for  two  assumed  lengths  of  drain— 1,000  feet  and  2,000 
feet.  Y^.k  is  1.5  feet,  that  is  one-half  of  depth  of  drain  where  the  soil  is  open  and 
saturated  with  water,  under  which  conditions  the  drain  will  discharge  its  maximum 
quantity.  Where  the  soil  is  close  no  additional  head  will  be  added  by  the  free 
water  of  the  soil,  so  that  the  factor  y2k  should  be  omitted  in  computations.  Three 
feet  of  soil  above  the  top  of  the  drain  has  been  assumed.  It  will  readily  be  seen  that 
the  grade,  length  of  drain  and  openness  of  soil  are  important  factors  in  the  capacity 
of  a  tile  drain  for  discharging  soil  water. 


SEWAGE    PURIFICATION    AND    DISPOSAL  141 

through  a  light,  sandy  soil  than  through  a  close,  tenacious 
clay.  The  common  distances  apart  are  20  to  30  feet  in 
very  compact  clay,  40  to  70  feet  in  average  loams  which 
have  an  open  subsoil,  from  100  to  200  feet  apart  in  very 
open  soils.  A  safe  distance  for  drainage  tiles  in  irrigation 
fields  of  average  loam  soils,  is  from  40  to  50  feet,  if  the 
depth  is  3^  feet  or  over.  Under  the  same  conditions,  "but 
for  heavy  clay  soils,  a  distance  of  25  to  40  feet  will  be 
found  safe. 

Grades  for  Irrigation  Underdrains — If  underdrain  tiles 
are  laid  at  too  low  a  grade  there  is  danger  of  depositing 
velocities  being  attained  which  will  silt  up  the  pipe,  pos- 
sibly in  course  of  time  effecting  a  complete  stoppage  of  the 
drain.  If,  on  the  other  hand,  drains  are  laid  at  too  great  a 
pitch,  they  will  give  equal  trouble,  but  from  a  different 
cause.  Twelve  inches  fall  in  100  feet  is  considered  about 
the  limit  of  safety.  If  drains  are  laid  at  a  steeper  grade, 
the  water  is  liable  to  attain  such  a  velocity  that  it  might 
loosen  the  tiles,  particularly  in  a  light  soil.  Three  inches 
in  100  feet  is  about  the  right  grade  for  underdrains.  When 
necessary,  however,  they  may  safely  be  laid  with  as  low 
grades  as  }h  inch  in  100  feet,  or  as  high  as  8  inches  to 
100  feet. 

The  tiles  should  be  laid  with  their  joints  as  close 
together  as  possible,  to  prevent  fine  materials  entering 
and  obstructing  the  drains.  There  is  no  danger  of  making 
the  joints  too  tight,  for  sufficient  space  will  always  remain 
to  admit  the  ground  water,  no  matter  how  close  the  tiles 
are  laid.  Most  tiles  are  warped  some  in  burning,  and, 
when  laying  them  on  the  bottom  of  ditches,  the  tiles 
should  be  turned  and  adjusted  until  they  have  not  only 
tight  joints  but  also  a  firm  bedding  from  which  they  cannot 
be  easily  displaced. 


METHODS  OF  IRRIGATING  WITH  SEWAGE 

Ridge  and  Furrow  Irrigation — One  method  of  surface 
arrangement  for  irrigation  by  the  ridge  and  furrow  system 


142 


SEWAGE    PURIFICATION    AND    DISPOSAL 


is  shown  in  Fig.  54.  Ridges  or  beds  of  any  convenient 
shape  and  size  are  laid  out  with  furrows  between.  Sewage 
is  discharged  into  these  furrows,  and,  by  force  of  capillary 
attraction,  reaches  all  the  roots  within  the  beds  or  ridges. 
What  moisture  is  not  required  by  the  roots  at  the  time  of 
applying,  the  sewage  seeps  into  the  subsoil,  where  it  remains 
in  storage  ready  to  be  drawn  upon  at  any  time.  Main  dis- 
tributors a,  which  may  be  of  pipe,  puddled  clay,  wood, 
concrete  or  any  convenient  material,  are  used  to  distribute 
sewage  to  the  various  beds.  At  the  various  beds,  sewage  is 
allowed  to  flow  through  sluice  gates  b  into  a  main  furrow, 
from  which  the  sewage  is  distributed  to  the  various  branch 


Fig.  54 

furrows  c  c.  Instead  of  having  ridges  and  furrows,  the 
surface  of  the  soil  may  be  made  level,  and  the  entire  field 
flooded  at  frequent  intervals.  That  would  be  the  better 
method  when  the  crops  are  of  grass,  nut  trees  or  like 
vegetation. 

Masonry  abutments  for  the  sluice  gates  are  shown  in 
this  system  of  distribution.  A  slightly  different  method,  in 
which  wooden  sluice  boxes  are  used,  is  shown  in  Fig.  55. 
Wooden  sluice  gates  or  paddles  are  likewise  used,  as  it  is 
not  necessary  that  they  be  absolutely  tight.  The  main 
consideration  in  designing  the  distributing  system  for 


SEWAGE    PURIFICATION    AND    DISPOSAL 


143 


sewage  irrigation  is  to  secure   durable,   but  inexpensive, 
flumes,  sluices  and  gates. 

Another  system  of  ridge  and  furrow  irrigation  is  shown 
in  Fig.  56.  This  system  is  adapted  to  land  having  a  slight 
slope  which  can  be  graded  into  a  series  of  terraces.  Sew- 
age is  applied  to  the  soil  in  this  system  by  flowing  in  a  thin 
film  over  the  surface  from  the  distributing  flumes  or  sluices 
a  a,  which  occupy  the  crest  of  the  ridges,  to  the  collecting 
'channels  b  b  in  the  furrows,  which  in  turn  discharge  into 
the  main  collecting  channel  c.  This  main  collecting  chan- 
nel c  answers  in  turn  as  a  distributing  main  for  the  terrace 
next  lower  down,  and  so  on,  until  the  entire  field  is  covered. 
In  the  illustration,  the  reference  letters  in  the  upper  and 


Fig.  55 


lower  terraces  are  reversed.  The  ridges  in  the  upper  ter- 
race which  are  marked  a,  in  the  lower  terrace  are  marked 
$,  and  the  furrows  of  the  upper  terrace  which  are  marked 
£,  are  marked  a  in  the  lower  terrace.  In  irrigating  by  this 
method,  the  ridge  channels  are  allowed  to  fill  with  sewage 
until  they  overflow  their  banks,  and  grooves  are  provided 
at  certain  intervals  in  the  branch  distributor,  as  shown  at 
d,  so  sewage  can  be  shut  off  from  the  ends  of  the  ridges 
when  sufficient  sewage  for  irrigation  has  overflowed,  at 
these  points,  and  the  dammed-up  sewage  made  to  overflow 
other  parts  of  the  field. 

In  constructing  irrigation  fields  according  to  this  system, 
the  beds  are  underdrained  and  are  laid  out  in  couples  with 
slopes  varying  from  i  foot  in  50  feet  to  i  foot  in  150  feet. 


144 


SEWAGE    PURIFICATION    AND    DISPOSAL 


A  clayey  soil,  being  less  porous  than  a  sandy  loam,  would 
require  a  lower  grade  in  order  to  absorb  sufficient  moisture 
while  the  film  of  water  is  passing  over.  Sandy  soil,  on  the 
other  hand,  will  wash  easier,  and  the  slope  will  have  to  be 
sufficiently  low  to  prevent  surface  washing.  Ordinarily  the 
beds  are  laid  out  with  a  total  width  of  30  to  40  feet,  half 
being  on  each  side  of  the  distributor,  and  a  length  of  from 
100  to  200  feet. 

This  method  of  laying  out  a  field  for  irrigation  is  more 
expensive  than  the  ridge  and  furrow  method  previously 


Fig.  56 

shown,  and  when  the  simpler  and  less  expensive  method 
is  practicable  it  is  the  better  to  use. 

Catchwork  System  of  Irrigation — A  catchwork  system 
of  irrigation  is  shown  in  perspective  in  Fig.  57.  In  this 
system,  which  is  specially  adapted  to  steep  hillsides,  the 
sewage  is  delivered  to  the  highest  part  of  the  area  to  be 
irrigated  through  a  main  carrier,  which  follows  the  contour 
of  the  land,  and  from  which  the  liquid  is  caused  to  overflow 
the  lower  edge  by  damming  the  main  distributor  at  various 
points.  At  various  elevations  lower  down  on  the  hillside 


SEWAGE    PURIFICATION    AND    DISPOSAL 


145 


are  collectors  or  gutters  following  the  contour.  The  sew- 
age from  the  main  distributor,  overflowing  from  the  lower 
side  in  a  thin  film,  flows  over  the  intervening  space,  and 
what  liquid  has  not  been  absorbed  by  the  soil  is  collected 
in  the  first  gutter.  When  this  collector  is  filled,  the  fluid 
overflows  its  lower  edge  as  in  the  case  of  the  main  dis- 
tributor and  flows  down  to  the  next  collector,  and  so  on  to 
the  last  gutter.  The  main  carrier  is  given  a  fall  of  about  2 
inches  in  100  feet,  so  the  farthest  end  of  the  field  will  be 
irrigated  first;  then  by  consecutive  damming  of  the  dis- 
tributor, each  part  of  the  hillside  will  be  successfully  flooded. 
The  main  distributor  is  made  from  8  to  10  inches  deep  and 
seldom  over  24  inches  in  width.  The  gutters  are  made 


Fig.  57 

level  throughout  their  length  and  great  care  must  be  exer- 
cised in  their  construction  so  all  parts  of  the  area  will 
receive  their  due  proportion  of  water. 

Quantity  of  Sewage  Required  for  Irrigation — The  quan- 
tity of  sewage  required  for  irrigation  depends  upon  the 
kind  of  crops  grown  and  the  rate  of  evaporation  from  the 
surface  of  the  soil.  As  the  raising  of  crops  is  the  chief 
consideration  in  irrigation,  no  more  sewage  should  be 
applied  than  is  actually  required;  if  the  limit  of  this 
requirement  is  exceeded  the  crops  will  be  destroyed.  In 


146  SEWAGE    PURIFICATION    AND    DISPOSAL 

arid  regions  where  evaporation  is  rapid,  more  sewage 
would  be  required  than  would  be  advisable  for  similar 
crops  in  a  more  favorable  place.  The  application  of  sew- 
age must  be  intermittent  to  allow  the  interstices  of  the  soil 
to  drain  and  aerate.  An  application  on  an  average  of  from 
5,000  to  15,000  gallons  of  sewage  per  acre  per  day  is  about 
all  most  crops  will  stand;  allowing  100  gallons  of  water 
per  capita  as  a  daily  average  consumption,  one  acre  of  land 
at  the  foregoing  rate  would  be  required  for  from  50  to  100 
persons.  The  general  data  concerning  a  number  of  British 
sewage  farms  are  condensed  into  Table  XIV.  This  table 
shows  the  high  degree  of  purification  of  effluents  which  can 
be  attained  by  sewage  irrigation,  the  character  of  the  sew- 
age and  rate  of  application  for  various  soils  that  will  give 
satisfactory  results  in  practice. 

Crops  for  Sewage  Farms — It  may  safely  be  stated,  as 
a  rule,  that  any  kind  of  vegetation  indigenous  to  the 
locality  can  successfully  be  raised  on  sewage-irrigated 
farms.  Among  the  crops  successfully  raised  on  sewage 
farms  now  in  operation  may  be  mentioned  root  plants,  like 
carrots,  parsnips,  potatoes  and  turnips ;  legumes,  like  beans 
and  peas ;  cereals,  like  oats,  barley,  wheat  and  corn ;  vege- 
tables, like  pumpkins  and  cabbage ;  soft-shell  English  wal- 
nuts, and  forage,  like  alfalfa  and  Italian  rye  grass.  The 
alfalfa,  however,  on  account  of  its  inclination  to  sod,  is 
found  difficult  to  cultivate.  The  raising  of  walnuts  has 
given  particularly  good  returns  for  the  money  invested. 


SUBSURFACE  IRRIGATION 

Principles  of  Subsurface  Irrigation— In  the  purification 
of  sewage  by  subsurface  irrigation,  sewage  is  applied  to  the 
soil  intermittently,  by  discharging  the  fluid  in  periodical 
doses  into, a  specially  constructed  system  of  distributing 
tiles,  buried  in  the  earth,  but  laid  close  to  the  surface  of 
the  ground.  The  trenches  in  which  the  tiles  are  buried 
are  filled  with  a  coarse  material  like  crushed  stone,  gravel 
or  cinders,  so  that  the  sewage  can  easily  escape  to  the 


SEWAGE    PURIFICATION    AND    DISPOSAL 


147 


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148  SEWAGE    PURIFICATION    AND    DISPOSAL 

surrounding  soil  and  there  be  attacked  by  the  reducing 
micro-organisms  in  the  presence  of  a  plentiful  supply  of  air. 
The  tiles  being  located  close  to  the  surface  and  surrounded 
by  porous  materials,  the  aerobic  bacteria  in  the  upper  strata 
of  soil  are  the  reducing  agents,  and,  like  in  intermittent 
nitration,  or  surface  irrigation,  the  process  is  more  of  an 
aerobic  than  anaerobic  purification;  consequently,  the 
process  is  free  from  the  putrefactive  odors  of  anaerobic 
fermentation. 

The  surface  of  the  ground  over  a  subsurface  disposal 
field  may  be  covered  with  a  top  dressing  of  loam  or  garden 
soil  and  laid  out  as  a  truck  garden,  lawn,  tennis  court, 
flower  garden,  or  may  be  put  to  any  other  ordinary  use 
without  interfering  with  the  disposal  process  beneath. 

Subsurface  irrigation  differs  from  surface  irrigation 
simply  in  the  method  of  applying  sewage  to  the  land.  To 
successfully  irrigate  by  the  subsurface  method,  the  land 
must  be  underdrained,  unless  there  is  a  sufficiently  low 
water-table ;  and  the  application  of  sewage  must  be  inter- 
mittent, as  constant  application,  even  in  a  small  stream, 
would  saturate  the  soil  without  purifying  the  sewage. 
Sewage  or  effluent  is  discharged  into  the  distributing  sys- 
tem intermittently  by  means  of  an  automatic  siphon, 
located  in  a  flush  tank  or  dosing  chamber,  depending  on 
whether  fresh  or  septic  liquid  is  to  be  applied  to  the  soil. 

The  subsurface  method  of  irrigation  is  the  least  satis- 
factory of  any  of  the  methods  of  irrigation  where  sewage 
is  applied  to  land,  but  the  method  can  be  used  without 
occupying  space  on  the  surface  of  the  ground,  for  which 
reason  it  is  preferred,  in  many  cases,  to  better  methods. 

Sewage  is  applied  to  the  soil  in  subsurface  irrigation 
through  drain  tiles  laid  with  open  joints,  at  depths  of  from 
12  to  1 8  inches,  beneath  the  surface  of  the  ground.  The 
trenches  in  which  the  tiles  are  laid  are  filled  in  around  the 
pipes  with  broken  stones  or  gravel,  to  allow  the  liquid  to 
flow  freely  from  the  drains  and  soak  into  a  large  area  of 
ground.  In  a  loose  soil  of  open  texture  the  liquid  is 
quickly  distributed  by  capillary  attraction  to  all  parts  of 


SEWAGE    PURIFICATION    AND    DISPOSAL  149 

n 


\£-MaifT  Draft?  to  Sewer  Oof f fa//. 
Fig.  58 

the  field  between  the  lines  of  distributing  tiles.  It  is  a 
good  practice  when  laying  the  distributing  tiles  to  turn  the 
end  of  each  branch  up  and  extend  it  to  the  surface  of  the 


150  SEWAGE    PURIFICATION    AND    DISPOSAL 

ground  to  admit  a  plentiful  supply  of  air  to  the  tiles  and 
voids  of  the  soil. 

Example  of  a  Subsurface  Disposal  Plant — An  example 
of  a  small  subsurface  disposal  plant  for  level  ground  is 
shown  in  Fig.  58.  This  plant  consists  of  two  parts,  the 
collecting  tank  and  dosing  chamber,  and  the  disposal  field. 
The  disposal  field  is  prepared  for  the  purpose  by  suitably 
underdraining  the  soil,  as  shown  by  the  dotted  lines,  and 
providing  a  system  of  subsurface  distributors,  through 
which  to  discharge  the  sewage  into  the  soil.  Any  modifi- 
cation of  this  arrangement  of  pipes  may  be  adopted.  For 
instance,  instead  of  having  the  distributing  main  at  one 
side  of  the  field  as  shown  in  the  illustration,  the  main  may 
be  located  in  the  center  of  the  disposal  field  with  branches 
taken  off  on  both  sides.  In  fact,  that  would  be  the  better 
plan  if  the  field  were  large.  Again,  instead  of  turning 
the  branches  at  right  angles  to  the  main,  as  was  done  in 
this  instance,  they  may  be  continued  from  the  branches  of 
the  Y's  at  angles  of  45  degrees. 

Instead  of  running  the  underdrains  at  right  angles,  to 
the  distributing  branches,  they  may  be  run  parallel  with 
them,  in  which  case  the  underdrains  should  be  spaced  so 
as  to  come  midway  between  two  distributing  branches 
instead  of  immediately  beneath  one.  In  case  the  under- 
drains are  run  as  indicated  on  the  drawings,  the  main 
branches  should  be  extended  to  the  sewer  manhole  with  the 
cleanout  plugs  located  within  so  the  mains  can  be  flushed, 
when  necessary,  direct  into  the  sewers. 

A  septic  tank  is  shown  in  this  plant.  A  septic  tank  is 
not  necessary,  however,  for  subsurface  sewage  disposal. 
Equal,  and  in  some  cases  better  results  can  be  obtained  by 
using  only  a  dosing  chamber  in  which  the  crude  sewage  is 
collected  until  sufficient  has  accumulated  to  dose  the  field. 
An  automatic  siphon  will  then  empty  the  contents  of  the 
tank  into  the  distributing  mains. 

Whatever  treatment  the  sewage  undergoes  before  being 
discharged  into  the  subsurface  tiles,  it  should  be  aerated  as 
fully  as  possible.  Sewage,  after  standing  for  some  time  in 


SEWAGE    PURIFICATION    AND    DISPOSAL 


151 


a  collecting  or  dosing  chamber,  also  the  effluents  from  sep- 
tic tanks,  are  wholly  devoid  of  air  or  oxygen,  and  as  the 
succeeding  operation  is  an  aerobic  one,  the  liquid  should  be 
discharged  into  the  underground  distributing  tiles  as  fully 
charged  with  air  as  it  is  possible  to  have  it  under  the 
circumstances. 

In  order  that  the  disposal  field  itself  and  the  interstices 
of  the  soil  be  freely  supplied  with  air,  the  ends  of  the 
distributing  tiles  should  be  turned  upward  and  extended  to 
the  surface  of  the  ground.  Indeed,  a  system  of  vents,  so 


Fig.  59 

arranged  that  air  can  circulate  through  the  disposal  tiles, 
will  be  found  to  increase  greatly  the  effectiveness  of  the 
process  and  raise  subsurface  irrigation  in  effectiveness 
almost  to  the  level  of  intermittent  filtration. 

Distributing  System  for  Subsurface  Irrigation — A  sec- 
tion of  the  distributing  system  for  the  foregoing  subsurface 
irrigation  plant  is  shown  in  detail  in  Fig.  59.  The  main 
distributor  is  of  salt-glazed  tile,  put  together  with  water- 
tight cemented  joints.  Each  distributing  main  is  controlled 
by  a  quick-closing,  lever-handle  shear  valve,  and  the  end 


152  SEWAGE    PURIFICATION    AND    DISPOSAL 

of  the  distributing1  main  is  plugged  with  a  stopper  which 
can    be   removed   at   any  time   to   flush  out  the   main. 

The  branch  fittings  for  the  distributing  branches  are 
made  eccentric  with  branch  outlets  flush  at  the  bottom, 
so  that  sewage  can  flow  freely  into  all  the  branches,  and 
the  mains  not  stand  part  full  of  the  liquid.  The 
branch  distributors  are  made  of  ordinary  drain  tiles  i  foot 
long  and  3  inches  in  diameter.  The  tiles  are  laid  with 
spaces  of  at  least  %  inch  between  the  ends,  which  rest  on 
earthenware  gutters  and  are  covered  with  earthenware 
caps  to  protect  the  joints. 

Method  of  Laying  Subsurface  Irrigation  'Tiles — The 
method  of  laying  subsurface  tiles  is  shown  in  Fig.  60.  A 
trench  is  dug  for  the  line  of  pipe  and  the  bottom  of  the 
trench  is  given  a  uniform  grade  of  not  more  than  i  inch  in 
50  feet.  On  the  bottom  of  this  trench  are  laid  the  distribu- 
ting tiles  a,  with  open  joints,  which  are  protected  from  the 
entrance  of  dirt  from  the  top  by  earthenware  caps  b  and 
rest  upon  the  gutters  c.  The  caps  and  gutters  are  made 
with  a  larger  radius  than  the  drain  tiles,  so  that  sewage 
can  escape  freely  from  the  joints  into  the  trench,  thence 

into  the  surrounding  soil. 
In  order  that  the  sewage 
may  move  freely  in  a  lateral 
direction,  and  thereby  fill  the 
entire  trench  to  the  normal 
level  of  the  sewage,  the 
trench  is  filled  in  around  the 
pipes  to  within  a  few  inches 
of  the  surface  of  the  ground 
with  crushed  stone,  broken 
bricks,  gravel,  cinders  or 
any  other  substance  which 
is  suitable  for  sprinkling  fil- 
ters. The  few  inches  of  space  above  the  coarse  materials 
may  be  filled  with  a  top  dressing  of  any  kind  suitable 
to  the  purpose  for  which  the  surface  will  .be  used. 

Fittings    for    Subsurface   Irrigation — Special   fittings, 


SEWAGE    PURIFICATION    AND    DISPOSAL  153 

which  differ  from  ordinary  sewer  pipe  fittings,  may  be  had 

for  subsurface   irrigation.     The   most   commonly  used   of 

subsurface  irrigation    fittings  is  the  Y  branch,   shown  in 

Fig.  6 1.     It  will  be  observed  that  the  branch  on  this  fitting 

is  hubless   and  is  taken  off   flush 

with  the  bottom  of  the  main  pipe, 

so  that  all  sewage  can  drain  from 

the  main  into  the  branch ;  further, 

the  hub  on  the  run  of  the  fitting  is  Fi&-  61 

on  the  opposite  end  from  that  of  an  ordinary  Y  fitting. 

Application  of  Subsurface  Irrigation  to  Hillside — The 
principles  of  subsurface  irrigation  on  a  hillside  are  the  same 
as  on  level  ground,  and  in  application  the  only  difference 
lies  in  the  manner  of  laying  the  tile.  On  a  hillside,  as  on  a 
level  plain,  the  tiles  must  be  laid  almost  level,  and  at  the 
same  time  be  within  a  certain  distance  of  the  surface.  To 
accomplish  this  on  hillsides,  which  have  not  perfectly  plain 
surfaces,  the  trenches  must  be  dug  more  or  less  crooked, 
following  the  contours,  to  keep  them  perfectly  level  and  at 
the  same  time  not  buried  too  deep. 

The  application  of  subsurface  irrigation  to  sloping 
ground  with  a  plain  surface  is  shown  both  in  plan  and  in 
elevation  in  Fig.  62.  It  will  be  noticed  that  at  every  branch 
from  the  distributing  main  the  line  of  pipe  is  stepped  down 
a  distance  corresponding  to  the  slope  of  the  ground  between 
branch  tiles.  In  this  case,  the  slope  of  the  ground  is  but  i 
inch  in  i  foot,  consequently  at  each  fitting  the  main  pipe 
is  stepped  down  3  inches,  the  branch  distributors  being 
spaced  3  feet  apart.  If,  instead  of  stepping  the  main 
drain  down,  as  shown  in  the  illustration,  it  were  sloped  or 
given  a  grade  similar  to  the  surface  of  the  land,  the  sewage 
would  flow  with  considerable  velocity  to  the  lower  end  of 
the  system  and  there  break  out  through  the  trenches, 
thereby  flooding  the  surface  of  the  ground.  By  running 
the  main  horizontal,  on  the  other  hand,  with  a  series  of 
steps  to  keep  it  below  the  surface  of  the  ground,  the  velocity 
of  the  liquid  is  retarded  and  the  upper  branches  receive 
their  shares  of  the  sewage  to  be  purified. 


154 


SEWAGE    PURIFICATION    AND    DISPOSAL 


In  installing  a  system  similar  to  this  on  a  hillside,  the 
distributing  tiles  should  be  run  near  the  top  of  the  trenches, 
leaving  plenty  of  space  filled  with  crushed  stone  or  other 
filtering  material  beneath  but  within  18  inches  of  the  sur- 
face of  the  ground.  If  the  distributing  tiles  were  laid  on 
the  bottom  of  the  trenches,  there  would  be  but  little  room 
for  sewage,  for  as  soon  as  the  liquid  rose  above  the  level  of 
the  bottom  of  the  branch  fitting,  there  would  be  a  head  of 
water  which  would  cause  it  to  flow  back  into  the  main 


Fig.  62 

drain,  thence  to  the  lower  end  of  the  field,  which  would 
become  flooded.  This  system  of  subsurface  irrigation  may 
be  put  in  without  the  branches  of  the  step-down  fitting 
extending  to  the  surface  of  the  ground  as  shown  in  the 
illustrations.  Indeed,  the  branch  which  extends  to  the 
surface  may  be  omitted  altogether.  However,  where  there 
is  no  objection  to  this  method  it  will  be  found  preferable, 
and,  if  the  ends  of  the  distributing  branches  are  likewise 
tiirned  up  to  the  surface,  there  will  be  a  circulation  of  air 
through  the  underground  pipes  and  absorption  ditches, 


SEWAGE    PURIFICATION    AND    DISPOSAL  155 

which  will  make  this  process  little  less  effective  than  surface 
application. 

Size  of  Absorption  Tiles — Absorption  tiles  for  subsur- 
face irrigation  are  usually  3  inches  in  diameter  and  i  foot 
in  length.  Smaller  pipes  would  be  unsatisfactory  on 
account  of  the  greater  liability  of  stoppage  and  because  the 
air  would  not  circulate  so  well  through  a  system  of  small 
pipes.  With  pipes  3  inches  in  diameter,  some  air  will  be 
present  even  when  the  fields  are  being  flooded. 

Depth  of  Absorption  Tiles — The  nearer  the  surface  of 
the  ground  that  absorption  tiles  can  be  placed  the  better 
will  be  the  aeration,  consequently  the  better  will  be  the 
purification  of  the  sewage.  In  fields  that  are  plowed,  how- 
ever, the  tiles  must  be  laid  a  sufficient  depth  below  the 
surface  so  they  will  not  be  displaced  by  the  plowshare. 
The  greatest  depth  that  absorption  tiles  are  laid  is  18  inches 
from  the  surface  of  the  soil  to  the  bottom  of  the  drain  tile. 
They  cannot  be  laid  in  a  trench  much  shallower  than  8 
inches  without  projecting  above  the  surface,  and,  ordinarily, 
are  laid  at  a  depth  of  12  inches  from  the  surface  of  the  soil 
to  the  bottom  of  the  trench. 

Grade  of  Absorption  Tiles — Absorption  tiles  should  be 
laid  almost  level,  or  at  most  at  a  grade  of  not  more  than  i 
inch  in  50  feet.  If  a  greater  grade  is  given  the  tiles,  the 
sewage  will  be  carried  to  the  lower  part  of  the  system, 
where  it  might  break  through  and  flood  that  part  of  the 
field,  while  the  upper  portion  remains  comparatively  dry. 
From  a  point  about  25  feet  away  from  the  absorption  tiles  to 
the  end  of  the  drain  the  main  drain  pipe  should  be  laid  with 
but  slight  fall,  so  the  sewage  will  not  be  carried  by  the 
momentum  to  the  lower  branches  of  the  system  to  the  ex- 
clusion of  the  upper  branches.  A  fall  of  i  inch  in  30  feet  will 
be  found  sufficient  for  the  main  drain. 

Distance  Apart  of  Absorption  Tiles — Absorption  tiles 
should  be  so  spaced  that  there  will  be  no  appreciable  fall  in 
the  water-line  between  any  two  branches  and  so  the  liquid 
seeping  laterally  will  reach  from  branch  to  branch.  In  a 
heavy  clay  soil  the  lateral  movement  will  be  very  slow,  so 


156  SEWAGE    PURIFICATION    AND    DISPOSAL 

that  the  tiles  cannot  be  spaced  far  apart,  while  in  an  open 
sandy  soil  the  downward  trend  of  the  sewage  would  prevent 
a  strong  lateral  movement  of  the  small  quantity  of  sewage 
applied,  so  that  in  soil  of  open  texture  the  distributing 
mains  cannot  be  spaced  far  apart.  In  practice  it  is  found 
that  a  distance  of  3  feet  will  prove  satisfactory  for  any  soil 
suitable  for  subsurface  irrigation  Any  greater  distance 
apart  would  greatly  reduce  the  size  of  dose  that  could  be 
applied  to  a  .given  area,  while  spacing  the  tiles  closer  to- 
gether would  greatly  increase  the  cost  of  a  plant  without 
adding  to  its  capacity  or  efficiency. 

Area  Required  for  Subsurface  Irrigation — The  area  of 
land  required  for  subsurface  irrigation  depends  largely  on 
the  mechanical  composition  of  the  soil,  its  texture  and 
whether  or  not  the  surface  of  the  land  will  be  used  for  any 
purpose.  It  is  obvious  that  a  coarse  soil  of  open  texture  to 
which  sewage  can  be  applied  twice  or  three  times  daily  will 
require  less  area  than  will  a  fine  soil  of  close  .texture  to 
which  sewage  can  be  applied  but  three  times  a  week.  Fur- 
ther, if  the  surface  of  the  land  is  used  as  a  lawn  or  for.  any 
other  purpose  which  would  require  that  it  be  kept  dry,  less 
sewage  could  be  applied  per  dose,  consequently  a  larger 
area  of  land  would  be  required  than  when  surface  wetting 
is  not  objectionable. 

As  a  matter  of  fact,  the  area  required,  size  of  dose  and 
character  of  soil  are  so  intimately  related  that  they  must 
be  considered  together  and  in  relation  to  one  another.  To 
obtain  satisfactory  results,  every  detail  of  a  subsurface  sys- 
tem should  be  worked  out  as  carefully,  or  more  so,  than  a 
surface  irrigation  plant,  for  in  case  of  failure  of  subsurface 
irrigation  there  is  danger  of  the  many  lateral  branches 
becoming  long,  shallow  cesspools.  Because  the  results  are 
hidden  from  view  in  this  method  of  disposal,  there  seems  to 
be  more  of  an  inclination  to  depend  on  empirical  propor- 
tions than  to  work  out  the  sizes  and  thus  get  accurate 
results. 

Take,  for  instance,  a  retentive  soil  of  close  texture, 
such  as  No.  6  in  Table  IX,  to  which  sewage  can  be  applied 


SEWAGE    PURIFICATION    AND    DISPOSAL  157 

but  three  times  in  seven  days.  If  in  such  a  soil  the  surface 
is  laid  out  for  a  lawn,  and  surface  wetting  would  be  objec- 
tionable, no  greater  dose  of  sewage  could  be  applied  than 
would  be  contained  by  the  absorption  trenches  without 
showing  above  the  surface  of  the  ground.-  An  acre  is  a 
square  Of  approximately  208  feet,  and  if  distributing  tiles 
were  spaced  3  feet  apart  in  an  acre  field  there  would  be 
approximately  14,000  lineal  feet  of  trenches  and  distributing 
branches.  Allowing  that  part  of  the  trenches  which  con- 
tains the  gravel,  stone  or  cinders  to  have  a  cross-section  of 
i  2  square  inches,  and  the  coarse  material  to  occupy  one-half 
the  space,  leaving  50  per  cent,  of  voids,  then  one  acre  of 
such  land  could  receive  at  one  dose  .5  x  14,000=7,000  cubic 
feet,  or  52,500  United  States  gallons  of  sewage.  But,  from 
the  mechanical  composition  of  the  soil,  that  is  a  greater 
quantity  than  could  be  cared  for  by  intermittent  filtration, 
which  is  limited  to  34,000  United  States  gallons  per  dose, 
and,  in  subsurface  irrigation,  at  least  10  per  cent,  less 
should  be  applied  than  to  intermittent  filter  beds.  If  the 
soil  were  similar  to  that  of  No.  4  or  5  in  Table  IX,  an  acre 
would  probably  care  for  the  full  52,500  gallons. 

The  foregoing  is  given  merely  to  show  the  necessity 
for  working  out,  from  the  character  of  the  soil,  the  area  of 
land  required  for  subsurface  irrigation. 

In  the  absence  of  definite  knowledge  of  the  soil,  empiri- 
cal formula  followed  in  practice  when  installing  plants  which 
have  not  been  designed  by  an  engineer  is  to  allow,  in  porous 
soil,  i  foot  of  3-inch  tile  pipe  for  each  gallon  of  sewage,  which 
is  equal  to  14,000  gallons  of  sewage  per  acre ;  and  in  clay  soils 
3  feet  of  3-inch  tile  are  allowed  for  each  gallon  of  sewage, 
which  is  equal  to  4,666  gallons  of  sewage  per  acre.  Accord- 
ing to  the  empirical  formula,  where  water  is  used  at  the 
rate  of  100  gallons  per  capita  per  day  one  acre  of  clayey 
land  would  be  sufficient  for  not  more  than  50  people,  while 
one  acre  of  sandy  soil  would  serve  for  140  people. 

Size  and  Frequency  of  Dose — The  size  and  frequency  of 
dose  will  depend  upon  the  mechanical  composition  or  texture 
of  the  soils,  the  same  as  for  intermittent  filtration  or  surface 


158  SEWAGE    PURIFICATION    AND    DISPOSAL 

irrigation.  Some  soils  will  take  a  dose  of  100,000  gallons 
or  more  per  acre,  applied  three  times  in  twenty-four  hours, 
while  other  soils  cannot  be  dosed  with  more  than  34,000 
gallons  at  a  time,  and  those  at  intervals  of  not  less  than 
fifty-six  hours.  In  order  to  properly  design  a  subsurface 
system,  a  mechanical  analysis  should  be  made  of  the  soils, 
if  sand,  or  they  should  be  classified  as  agricultural  lands,  if 
loams.  Knowing  the  composition  of  the  soil,  the  size  of 
dose  and  frequency  of  application  suitable  for  that  soil  can  be 
found  either  in  Table  IX,  which  gives  the  quantity  of  sewage 
purified  by  sands  of  different  mechanical  compositions  and 
the  frequency  with  which  sewage  can  be  applied,  or  Table 
XIV,  which  shows  the  amount  of  sewage  which  can  safely 
be  applied  to  agricultural  soils  in  sewage  irrigation.  Larger 
doses  or  more  frequent  applications  than  those  stated  in 
the  tables  should  not  be  resorted  to  if  satisfactory  results 
are  desired.  Indeed,  on  account  of  the  underground 
method  of  applying  the  sewage,  better  results  will  be 
obtained  by  allowing  from  10  to  25  per  cent,  less  for  a  dose. 

The  empirical  method  followed  in  practice,  where  no 
consideration  is  taken  of  the  composition  of  the  soil,  is  to 
allow  for  one  flooding  in  twenty-four  hours.  When  the 
size  of  dose  used  is  small,  this  method  will  no  doubt  give 
fairly  good  results. 

Underdrains  for  Subsurface  Irrigation — Fields  for  sub- 
surface irrigation  should  be  underdrained  the  same  as  for 
intermittent  filtration  or  sewage  irrigation,  unless  there  is 
a  sufficiently  low  water-table  to  take  care  of  the  drainage 
naturally.  For  this  purpose,  the  method  of  underdraining 
agricultural  lands  for  sewage  irrigation  will  be  found  satis- 
factory, and  what  has  already  been  said  on  that  subject  will 
apply  equally  to  the  underdrainage  of  subsurface  irriga- 
tion fields. 

Operation  of  Subsurface  Irrigation  Plants — In  opera- 
tion, subsurface  irrigation  fields  are  dosed  daily  for  one 
week.  Sewage  is  then  shut  off  from  that  field  and  it  is 
allowed  a  period  of  rest  of  from  one  week  to  ten  days, 
depending  on  the  condition  of  the  soil  and  the  reserve 


SEWAGE    PURIFICATION    AND    DISPOSAL  159 

irrigation  fields.  Usually  two  irrigation  fields  are  provided, 
but  where  there  will  be  a  constant  and  uniform  quantity  of 
sewage  during  the  entire  year  it  will  be  found  advisable  to 
provide  three  fields.  The  surface  of  fields  over  absorption 
drains  can  be  laid  out  as  a  lawn,  flower  garden  or  vegetable 
garden;  no  vegetation  should  be  planted,  however,  the 
roots  of  which  would  be  liable  to  reach  down  into  the  tiles 
and  clog  the  pipes. 

Subsurface  irrigation  is  not  seriously  affected  by  frost, 
and  can  successfully  be  employed  in  regions  where  snow 
covers  the  ground  during  the  winter  months,  provided  the 
mean  temperature  of  the  air  is  not  below  18  degrees  Fahren- 
heit. This  method  of  purifying  sewage  is  not  used  exten- 
sively on  a  large  scale  for  villages  or  cities,  but  finds  its 
greatest  application  in  the  caring  for  domestic  wastes  from 
isolated  houses  where  there  are  no  sewer  systems ;  hospitals, 
sanatoriums,  prisons,  asylums,  country  and  seaside  hotels, 
summer  resorts  and  like  institutions,  located  in  country  or 
suburban  places.  If  properly  designed  and  installed,  sub- 
surface irrigation  will  give  very  satisfactory  results,  but 
will  be  found  more  expensive  to  install  than  many  other 
methods,  and  is  liable  to  be  the  source  of  trouble  and 
expense,  owing  to  the  clogging  of  the  tiles  by  roots  of 
plants,  which  enter  the  joints  in  search  of  water. 


ANTISEPTIC    TREATMENT    OF    SEWAGE 


CHEMICAL  PRECIPITATION  OF  SEWAGE 

Principles  of  Chemical  Precipitation — If  sewage  be 
stored  in  a  tank  for  a  certain  period  of  time,  or  passed 
continuously  through  the  tank,  but  at  a  low  velocity,  a 
certain  proportion  of  the  matter  carried  in  suspension  will 
be  deposited  on  the  bottom  of  the  tank  in  the  form  of 
sludge,  while  particles  of  lighter  specific  gravity  than 
water  will  float  to  the  top.  In  that  case  the  separation 
which  takes  place  within  the  tank  is  due  entirely  to  sedi- 
mentation, the  particles  having  reached  their  various  levels 
in  accordance  with  the  laws  of  gravity.  With  sewage, 
however,  which  contains  substances  differing  widely  in 
their  specific  gravities,  the  process  of  sedimentation  is  slow 
and  inefficient,  and  can  be  increased,  likewise  the  period 
of  storage  shortened,  by  adding  to  the  sewage  some  sub- 
stance which  will  produce  a  flocculent  precipitant  to  hasten 
sedimentation.  The  precipitant  in  settling  to  the  bottom 
of  the  tank  gathers  together  and  holds  whatever  suspended 
matter  it  encounters,  thus  effecting  a  partial  clarification 
of  the  sewage.  The  precipitation  of  sludge  by  means  of 
chemicals  cannot  be  considered  a  purification  process  of 
the  same  rank  as  bacterial  purification,  for  the  entire 
organic  content  of  the  sewage  still  remains  in  the  tank  and 
must  be  removed  or  undergo  the  usual  processes  of  fer- 
mentation and  putrefaction  before  reaching  a  stable  condi- 
tion. However,  a  certain  amount  of  clarification  takes 
place,  due  to  the  mechanical  separation  of  the  suspended 
matter,  and  as  the  effluent  contains  less  organic  matter 
than  the  sewage,  the  process  may  be  considered  a  purifica- 
tion. Usually  the  reagents  used  to  precipitate  sewage  are 
of  an  antiseptic  nature,  and  as  the  object  of  sewage  purifi- 
cation is  the  ultimate  destruction  or  resolution  of  the 
organic  matter  into  other  combinations,  it  is  evident  that 
an  antiseptic  process  is  the  very  reverse  of  the  end  to  be 
attained. 


SEWAGE    PURIFICATION    AND    DISPOSAL  161 

Treatment  of  sewage  by  chemical  precipitation  usually 
consists  of  two  stages;  the  precipitate  must  first  be  inti- 
mately mixed  with  the  crude  sewage,  and  the  mixture 
must  then  be  allowed  a  period  of  sedimentation  in  which 
to  separate  the  sludge  from  the  liquid.  Chemical  precipi- 
tation is  not  a  suitable  process  for  municipalities  or  country 
institutions  only  under  exceptional  conditions.  Its  chief 
claim  to  recognition  lies  in  the  value  it  might  possess  as  a 
preliminary  treatment  for  industrial  wastes. 

To  be  successfully  treated  by  chemical  precipitation, 
the  sewage  should  be  delivered  at  the  precipitation  works 
in  a  fresh  condition.  It  is  only  the  matter  in  suspension 
which  is  removed,  and  if  the  sewage  has  progressed  so  far 
that  most  of  the  organic  matter  is  in  solution,  very  little 
clarification  can  be  effected.  Liberal  tankage  should  be 
provided  for  the  sewage,  and  sludge  should  be  removed 
from  the  tanks  at  frequent  intervals  and  before  putrefac- 
tion sets  in. 

Precipitants — Within  certain  limits,  the  results  ob- 
tained by  the  use  of  a  reagent  depend  upon  the  care  and 
skill  with  which  it  is  prepared,  and  the  thoroughness  and 
proportion  of  its  admixture.  There  is  a  certain  amount  of 
any  chemical  used  as  a  precipitant  which  gives  the  best 
results  with  the  quality  of  sewage  treated,  and  if  less  than 
that  amount  be  used,  or  it  be  indifferently  mixed,  there 
will  be  less  clarification,  consequently  less  formation  of 
sludge;  on  the  other  hand,  if  too  great  a  quantity  be  used, 
and  it  be  improperly  mixed,  some  portion  of  the  chemical 
will  escape  with  the  efHuent.  Many  chemicals  could  be 
used  as  precipitants  for  sewage,  but  there  are  not  many 
which  fill  the  requirements  of  being  chemically  safe,  non- 
injurious  to  fish,  that  will  not  greatly  increase  the  amount 
of  sludge,  and  that  can  be  obtained  in  any  locality  at  low 
cost.  The  selection  of  a  reagent  will  depend  greatly  on 
the  availability  of  the  several  suitable  chemicals  at  the 
point  of  use.  In  Table  XV  will  be  found  several  chemicals 
and  the  results  obtained  by  their  use,  singly  and  in  combi- 
nation, in  experiments  conducted  by  the  Massachusetts 


162 


SEWAGE    PURIFICATION    AND    DISPOSAL 


State  Board  of  Health.  The  quantities  of  reagents  used  in 
the  table  are  comparative,  to  show  the  clarification  effected 
by  using  an  amount  of  various  chemicals,  which  at  the 
time  of  the  experiments  would  cost  30  cents  per  year  for 
treating  100  gallons  of  sewage  daily.  As  a  matter  of  com- 
parison, the  results  obtained  by  allowing  the  same  sewage 
to  settle  for  one  hour,  and  the  general  results  obtained  by 
intermittent  filtration  are  likewise  included. 

On  account  of  the  variation  in  the  composition  of  dif- 
ferent sewages,  no  one  precipitant  can  be  said  to  be  better 
than  another.  Lime,  which  has  been  extensively  used  as  a 
precipitant,  is  objectionable  on  account  of  the  sludge  it 

TABLE  XV — REAGENTS  FOR  PRECIPITATING  SEWAGE 


Chemical  Used 

Quantity  of 
Sewage  Treated 
Gallons 

Period  of 
Sedimenta- 
tion 
Hours 

Percentage  of 
Albuminoid 
Ammonia 
Removed 

1,800  pounds  of  lime    . 
650  pounds  of  alum      .     . 
1,000  pounds  of  copperas 
and  700  pounds  of  lime 
270  pounds  of  ferric  oxide 
in  the  form  of  ferric  sul- 
phate      .               .     •     • 

1,000,000 
1,000,000 

1,000,000 
1  000  000 

1 
1 

1 
1 

52 
51 

57 
59 

Plain  sedimentation 

1 

21 

Filtered  intermittently 
through  5  feet  of  sand  . 

98 

forms.  Alum  sulphate  and  ferric  sulphate  give  the  best 
results  under  general  conditions.  The  results  with  ferric 
sulphate  are  on  the  whole  more  satisfactory  than  those 
with  alum  sulphate.  The  effluents  from  sewage  treated 
with  iron  salts,  however,  are  slightly  colored  and  under 
some  conditions  might  be  objectionable.  It  might  be 
stated  that,  as  a  rule,  ferric  salts  are  preferable  to  ferrous 
salts  by  reason  of  their  quicker  action  and  more  insoluble 
precipitate,  and  within  certain  limits  the  more  of  either 
ferric  or  ferrous  sulphate  used  the  better  will  be  the  result 
obtained.  In  adding  chemicals  to  the  crude  sewage,  a 
solution  is  preferable  to  solids.  When  the  chemical  is  used 


SEWAGE    PURIFICATION    AND    DISPOSAL  163 

in  the  form  of  a  solution,  care  must  be  exercised  to  secure  a 
thorough  dissolution  of  all  the  substance  in  the  mixing 
tanks,  and  to  secure  a  uniform  strength  of  solution.  The 
variation  of  flow  in  sewage  from  hour  to  hour  also  calls  for 
constant  watchfulness  on  the  part  of  attendants,  unless  some 
automatic  device  is  used  to  proportion  the  amount  of  chemi- 
cal to  the  constantly  changing  conditions.  When  the  sew- 
age flows  into  the  settling  tanks  by  gravity,  the  chemicals 
can  be  added  to  the  sewage  in  a  mixing  channel  before  it 
reaches  the  tanks.  The  chemical  may  be  discharged  into 
the  sewage  through  a  number  of  perforations  in  a  pipe 
extending  across  the  channel,  or  from  a  notched  trough 
from  which  the  solution  flows  in  a  number  of  small  streams. 
Usually  a  mechanical  stirrer  is  provided  to  still  further  mix 
the  chemical  with  the  sewage.  Sometimes  a  water-wheel, 
operated  by  the  sewage,  is  used  for  this  purpose,  while  in 
other  cases  baffle  plates  and  deflecting  boards  are  employed. 
In  plants  where  the  sewage  must  be  pumped,  the  chemical 
can  be  introduced  to  the  pump  well,  the  action  of  the  pump 
being  depended  on  to  thoroughly  mix  the  chemical  with 
the  sewage. 

EXAMPLE  OF  A  CHEMICAL  PRECIPITATION  PLANT 

Continuous=Flow  Chemical  Precipitation  Tanks — In  the 

continuous-flow  process  of  chemical  precipitation,  crude 
sewage,  to  which  a  precipitant  has  been  added,  is  allowed 
to  flow  slowly  through  a  tank  and  discharge  over  a  weir  into 
the  outlet  chamber.  A  pair  of  continuous-flow  precipita- 
tion tanks  is  shown  in  perspective  in  Fig.  63.  In  the  sluice 
way  to  the  tanks,  baffle  plates  are  placed  so  as  to  stir  up 
and  agitate  as  much  as  possible  the  chemically  treated 
sewage.  Each  tank  is  provided  with  an  inlet  sluice  gate, 
so  sewage  can  be  cut  off  from  either  tank.  The  inside  of 
each  tank  is  divided  by  means  of  walls  or  partitions  into  a 
series  of  channels  through  which  the  chemically  treated 
sewage  must  slowly  flow  before  it- can  reach  the  outlet 
weir.  During  this  slow  passage  through  the  tank,  the 


164 


SEWAGE    PURIFICATION    AND    DISPOSAL 


heavy  particles  of  matter  will  be  carried  to  the  bottom,  while 
those  lighter  than  water  will  float  on  the  top  and  be  held  back 
by  the  baffle  wall  built  in  front  of  the  weir.  The  bottoms  of 
the  tanks  are  sloped  toward  one  point,  where  are  located 


5 fudge  P/pe 


Outlet 


5/udge  Pipe 


Fig.  63 


valved  sludge  outlets,  which  may  be  connected  to  sludge 
wells,  sludge  beds  or  other  place  of  sludge  disposal.  In 
case  the  sludge  is  to  be  handled  by  manual  labor,  and  the 
tanks  are  at  such  an  elevation  that  gravity  discharge  of 


SEWAGE    PURIFICATION    AND    DISPOSAL  165 

sludge  is  impossible,  the  sludge  pipes  and  sluice  gates  may 
be  omitted. 

In  large  chemical  precipitation  plants,  manual  labor  is 
dispensed  with  as  much  as  possible,  and  machinery  is  used 
to  perform  the  heavy  and  laborious  work.  A  building  is 
necessary  in  which  to  house  this  machinery,  and  in  the 
building  provision  must  be  made  for  the  various  apparatus. 
In  the  first  place,  tanks  must  be  provided  to  mix  the 
reagents  in;  sludge  wells  are  required  in  which  to  drain 
the  sludge;  pumps  are  required  for  handling  the  sludge, 
and  sludge  presses  or  other  devices  for  removing  the  water 
from  the  sludge  so  as  to  make  it  less  difficult  to  handle.  If 
sludge  presses  are  used,  air  compressors  and  storage  tanks 
will  likewise  be  required  for  operating  the  presses,  and 
instead  of  sludge  pumps,  compressed  air  sewage  ejectors 
may  be  used  for  elevating  the  sludge  from  the  sludge  wells 
to  the  presses. 

In  short,  there  are  many  auxiliary  parts  that  will  be 
required  for  a  fully  equipped  chemical  precipitation  plant. 
Just  what  those  auxiliary  parts  will  be  is  impossible  to 
state,  as  they  will  depend  in  each  case  on  the  individual 
opinions  of  the  designer.  All  that  is  necessary  here  is  to 
point  out  the  necessity  for  some  kind  of  machinery  for 
operating  the  plants  and  a  building  to  house  the  machinery 
in.  At  some  chemical  precipitation  plants,  the  tanks  are 
housed  in  covered  sheds,  while  in  others  they  are  exposed 
to  the  weather. 

Fill~and=Draw  Chemical  Precipitation  Tanks — Instead 
of  the  continuous-flow  method,  fill-and-draw  precipitation 
is  sometimes  resorted  to,  although  the  continuous-flow 
method  is  generally  considered  the  better  of  the  two.  In 
the  fill-and-draw  method,  the  sewage  is  treated  with  a 
chemical,  stored  in  a  tank  for  a  certain  period  of  time,  sel- 
dom exceeding  four  hours,  and  is  then  withdrawn.  If  the 
continuous-flow  tanks  shown  in  Fig.  63  had  their  interior 
partitions  removed,  and  draw-off  sluice  gates  provided, 
they  would  answer  very  readily  for  fill-and-draw  tanks. 

Usually,  chemical  precipitation  tanks  are  used  in  series. 


166  SEWAGE    PURIFICATION    AND    DISPOSAL 

That  is,  the  sewage  flows  from  the  mixing  channel  into 
primary  tanks  known  as  "roughing  tanks,"  where  the 
heaviest  particles,  amounting  perhaps  to  75  per  cent,  of  the 
sludge,  settle,  while  the  remaining  25  per  cent,  remains  in 
suspension  and  is  carried  over  the  weir  into  the  finishing 
tanks.  In  these  finishing  tanks  the  sludge  which  is  carried 
in  suspension  from  the  roughing  tanks  settles,  and  the  top 
water,  which  is  comparatively  clear,  flows  over  the  weirs 
into  the  effluent  channels. 

Floating  Outlet  lor  Decanting  Effluents — Each  chemical 
precipitation  tank  should  be  provided  with  a  valved  floating 
outlet,  connected  to  the  effluent  pipe,  so  that  the  clarified 
liquid  can  be  removed  from  the  tank  without  disturbing  the 
sludge.  Then,  when  it  is  desired  to  empty  a  tank,  the 
float  effluent  gate  can  be  opened  and  the  supernatant  liquid 
drawn  off  through  the  floating  outlet  into  the  effluent  chan- 
nel. When  the  clarified  water  has  been  lowered  to  the 
surface  of  the  sludge,  the  floating  effluent  gate  can  be 
closed,  the  sludge  gate  opened,  and  the  sludge  allowed  to 
flow  by  gravity  to  the  sludge  well  or  sludge  filters. 

Capacity  of  Precipitation  Tanks — Chemical  precipitation 
is  so  little  used  in  the  United  States  that  no  definite  pro- 
portions have  been  worked  out  for  the  sizes  of  tanks 
required.  There  should  always  be  a  sufficient  number  of 
tanks,  however,  so  that  one  can  be  thrown  out  of  service 
for  cleaning  and  repairs  without  interfering  with  the  oper- 
ation of  the  rest  of  the  plant.  The  size  of  tanks  will  depend 
somewhat  upon  the  period  allowed  for  sedimentation  and 
whether  rain  water  is  discharged  into  the  drainage  system. 
In  intermittent  precipitation  tanks,  the  period  of  sedimenta- 
tion is  approximately  two  hours,  although  four  to  six  hours 
will  effect  greater  clarification ;  and  in  chemical  precipita- 
tion tanks  where  rain  water  is  excluded,  and  one  hour  sedi- 
mentation provided  for,  a  gross  tank  capacity  of  40  per 
cent,  of  the  daily  flow  will  doubtless  prove  sufficient.  The 
tanks  should  be  deep  enough  so  the  precipitated  matter  will 
not  be  stirred  up  by  surface  currents  and  not  so  deep  as  to 
require  a  specially  long  time  for  the  precipitated  matter  to 


SEWAGE    PURIFICATION    AND    DISPOSAL  167 

reach  the  bottom.  Tanks  from  5  to  6  feet  deep  seem  to  fill 
both  these  requirements. 

Sludge  in  Chemical  Precipitation  Tanks— The  disposal 
of  sludge  is  the  most  serious  problem  connected  with 
chemical  precipitation.  Ordinarily,  the  wet  sludge  will 
amount  to  approximately  24  tons  for  every  million  gallons 
of  sewage  treated.  The  wet  sludge  contains  approximately 
5  to  10  per  cent,  of  solids  and  90  to  95  per  cent,  of  water. 
After  being  air  dried  or  passed  through  a  filter  press,  the 
organic  matter  amounts  to  from  26  to  30  per  cent. 

At  Worcester,  the  pressing  and  disposal  of  sludge  costs 
over  five  dollars  per  million  gallons  of  sewage  treated,  and 
to  this  amount  must  be  added  the  cost  of  chemicals,  labor 
and  operation  of  the  plant.  Owing  to  the  cost  of  chemicals 
the  bother  and  cost  of  disposing  of  the  sludge  and  the  poor 
effluent  which  is  liable  subsequently  to  putrefy,  this  method 
is  suitable  only  as  a  preliminary  process  to  be  followed  by 
a  biological  treatment,  and  even  then  it  is  satisfactory 
only,  as  was  previously  stated,  for  treating  industrial 
wastes. 


SEWAGE   PUMPING   PLANTS 


%     GENERAL  CONSIDERATION 

Many  conditions  arise  in  the  sewering  of  cities  or  in 
the  installation  of  purification  plants,  which  necessitate 
the  pumping  of  sewage.  To  do  so,  the  sewage  is  collected 
in  a  receiving  well  or  tank,  from  which  it  is  pumped  to  the 
point  of  disposal.  The  wells  are  usually  made  in  duplicate, 
and  each  well  contains  sufficient  capacity  for  several  hours' 
flow  of  sewage,  while  the  pumps,  unless  they  are  operated 
by  means  of  a  directly  connected  motor,  are  cross-con- 
nected so  that  either  or  both  can  be  operated  by  either  or 
both  of  the  prime  movers. 

Centrifugal  pumps  are  found  the  most  satisfactory  for 
pumping  sewage,  and  for  this  reason  are  generally  used. 
Almost  anything  which  will  enter  the  inlet  port  of  a  cen- 
trifugal pump  can  be  discharged  through  the  force  main, 
so  that,  as  a  rule,  there  is  little  trouble  experienced  with 
centrifugal  pumps  due  to  clogging.  There  is  a  notable 
exception  to  this  statement,  however.  Cotton  waste,  when 
introduced  to  the  drainage  system,  often  finds  its  way  to 
the  centrifugal  pump  and  interferes  with  its  proper  action. 
For  this  reason  screens  should  be  provided  in  sewage  wells, 
to  catch  or  hold  back  any  large  solid  matter  or  fibrous 
material  like  cotton  waste  that  might  interfere  with  the 
operation  of  the  pumps. 

Centrifugal  Pumps — Centrifugal  pumps  are  best 
adapted  for  raising  large  quantities  of  liquid  against  low 
heads.  For  this  reason  they  are  found  particularly  suitable 
for  pumping  sewage.  By  the  use  of  multiple  stage  centrif- 
ugal pumps,  liquids  can  be  raised  to  almost  any  reasonable 
height,  but  for  ordinary  work  the  multiple  stage  pump  will 
not  be  required,  the  single  stage  proving  entirely  satisfac- 
tory. Single  stage  pumps,  however,  should  not  be  used 
when  the  sewage  must  be  raised  a  greater  height  than  150 
feet.  The  open  impeller  pump  is  better  suited  to  sewage 
pumping  than  is  the  closed  impeller,  notwithstanding  the 
fact  that  closed  impeller  pumps  possess  about  4  per  cent. 


SEWAGE    PURIFICATION    AND    DISPOSAL  169 

greater  efficiency  than  equal  sizes  of  open  impeller  pumps. 
Centrifugal  pumps  cannot  be  economically  used  when  the 
quantity  of  sewage  to  be  moved  is  less  than  100  gallons  per 
minute. 

The  United  States  Drainage  Commission  tests  have 
shown  that  to  obtain  the  best  results  the  water  entering 
centrifugal  pumps  should  have  a  velocity  of  not  over  8  feet 
per  second  and  discharge  velocities  of  about  1 2  feet  per 
second.  To  produce  those  velocities  in  a  well  designed 
centrifugal  pump,  the  impeller  blades  should  have  a  cir- 
cumferential velocity  of  approximately  50  feet  per  second. 
This  will  produce  a  whirl  velocity  through  the  discharge 
of  the  impellers  of  from  30  to  40  feet  per  second,  which 
must  be  slowed  down  to  1 2  feet  per  second  or  less  in  the 
discharge  pipe  from  the  pump.  To  accommodate  the  vari- 
ous velocities  in  a  centrifugal  pump,  the  suction  and  dis- 
charge ports  should  be  provided  with  taper  connections, 
and  no  check  valves  not  absolutely  required  should  be  used. 

The  submerged  type  of  centrifugal  pump  is  the  kind 
best  suited  to  pumping  sewage.  The  pump  is  then  located 
on  the  floor  of  the  receiving  well,  and  the  shafting  extends 
to  the  floor  above  where  the  driving  mechanism  is  located. 
Pumps  having  discharge  pipes  of  less  than  2  inches  diame- 
ter are  not  suitable  for  sewage  work,  and  for  municipal 
plants  2^  inches  should  be  the  minimum  limit.  It  maybe 
stated  that  the  average  commercial  efficiency  of  the  large 
size  multiple  stage  turbine  pump  is  about  65  per  cent.,  and 
that  of  equal  sizes  of  straight  single  impeller,  volute,  cen- 
trifugal pump,  working  against  heads  less  than  100  feet,  is 
80  per  cent,  of  the  theoretical  efficiency. 

The  efficiency  of  centrifugal  pumps  increases  with  the 
diameter  of  the  suction  and  discharge  pipes.  For  instance, 
the  efficiency  of  a  2-inch  pump  is  about  38  per  cent. ;  of  a 
3-inch  pump,  45  per  cent. ;  of  a  4-inch  pump,  52  per  cent.  ; 
of  a  5-inch  pump,  60  per  cent.  ;  of  a  6-inch  pump,  64  per 
cent.;  of  an  i8-inch  pump,  77  per  cent.,  and  of  a  32-inch 
pump,  80  per  cent.  If,  however,  it  is  assumed  that  the 
size  of  pump  used  in  average  small  installments  will  be 


170 


SEWAGE    PURIFICATION    AND    DISPOSAL 


somewhere  between  3  inches  and  8  inches  in  diameter,  it 
can  safely  be  assumed  that  the  pump  will  not  develop  a 
greater  efficiency  than  55  per  cent. 

The  capacity  in  gallons  per  minute  of  centrifugal 
pumps  operated  at  different  velocities  may  be  seen  in  Table 
XVI.  It  should  be  remembered,  however,  that  when  de- 
signing a  pumping  plant  the  most  economical  velocity  is 
approximately  12  feet  per  second,  and  the  pump  which  will 
handle  the  required  volume  at  this  or  a  less  velocity  should 
be  used.  By  so  doing,  not  only  will  economy  be  obtained 
in  the  operation  of  the  plant,  but  sufficient  reserve  pump- 
ing capacity  will  be  available  to  handle  any  ordinary  flow 
of  sewage. 

TABLE  XVI— CAPACITIES  OF  CENTRIFUGAL  PUMPS 


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Revolutions  per  Minute 

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9 

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6 

425 

590 

680 

725 

825 

900 

975 

1050 

1120 

1170 

300 

2 

7 

400 

450 

525 

575 

650 

720 

780 

852 

908 

960 

650 

3 

7 

350 

400 

425 

450 

500 

550 

650 

775 

850 

910 

1250 

4 

10 

275 

300 

350 

400 

450 

500 

600 

675 

800 

890 

2600 

6 

12 

200 

220 

240 

300 

360 

420 

490 

540 

580 

610 

4750 

8 

15 

185 

200 

225 

250 

310 

360 

390 

425 

450 

475 

7500 

10 

18 

166 

188 

220 

245 

285 

320 

360 

386 

414 

436 

Methods  of  Driving  Sewage  Pumps— The  method  of 
driving  sewage  pumps  must  be  determined  in  each  case 
from  the  data  at  hand.  If  the  pumping  station  be  a  small 
one,  and  worked  intermittently,  electricity  might  be  advis- 
able. If  adjoining  a  power  house  where  steam  can  be 
obtained  at  a  reasonable  price,  steam  engines  might  prove 
the  most  economical.  When,  however,  the  pumping  plant 
is  isolated  from  other  buildings,  as  is  usually  the  case  at 
purification  plants,  it  will  be  found  necessary  to  have  an 
engineer  in  charge,  and  under  such  conditions  gasoline  or 
kerosene  engines  will  prove  the  most  economical. 


EXAMPLES  OF   PUMPING   PLANTS 


ELECTRIC  PUMPING  PLANT 

The  interior  of  a  pumping1  station  which  is  operated 
electrically  is  shown  in  Fig.  64.  In  this  plant,  three  pumps 
are  employed,  each  of  which  is  located  in  a  separate  sewage 
well,  and  is  direct-connected  by  vertical  shafting  to  an 


172  SEWAGE    PURIFICATION    AND    DISPOSAL 

electric  motor  in  the  room  above.  A  slip  coupling  is  placed 
in  each  shaft  just  below  the  floor  and  the  weight  of  the 
motor  and  its  shaft  is  carried  by  the  motor  bearing,  while 
the  weight  of  the  pump  shaft  and  the  thrust  of  the  im- 
pellers are  supported  by  a  thrust  bearing,  set  on  a  pair  of 
I-beams,  which  likewise  serve  as  guides  for  the  shaft.  A 
slip  coupling  is  found  useful,  as  it  permits  the  close  adjust- 
ment of  the  impeller  to  the  bottom  of  the  pump  casing, 
which  is  necessary  to  prevent  the  clogging  of  the  pump  by 
small  rags  of  cotton  waste  winding  around  the  shaft.  Fur- 
thermore, it  provides  for  the  adjustment  of  the  motor  and 
thrust  bearing,  so  that  each  will  do  its  share  of  the  work. 

A  thrust  bearing  is  desirable  in  connection  with  verti- 
cal shaft  direct-connected  pumps,  and  one  should  be  selected 
in  which  the  oil  is  continuously  and  automatically  circulated. 

The  pumps  in  such  a  plant  can  all  be  worked  inter- 
changeably or  they  may  be  adjusted  to  start  when  the 
sewage  reaches  different  elevations  in  the  several  wells. 
For  instance,  the  pump  in  the  first  well  can  be  adjusted  to 
start  into  operation  when  the  sewage  reaches  an  elevation 
of  5  feet;  that  in  the  second  well  when  the  liquid  raised  to 
8  feet,  and  the  last  pump  can  be  brought  into  service  when 
the  sewage  reached  the  lo-foot  mark.  By  this  means, 
smaller  units  can  be  used,  and  ample  reserve  capacity  will 
always  be  available  to  care  for  unusual  conditions  when 
there  is  an  exceptional  flow  of  sewage.  Such  a  plan  will 
be  found  very  satisfactory  for  summer  resorts  or  other 
places  where  the  population  varies  greatly  during  the  year, 
some  months  being  several  times  as  large  as  at  other 
seasons.  One  of  the  pumps  can  then  be  used  for  the  per- 
manent population  during  the  closed  season  and  the  other 
pumps  brought  into  service  as  occasion  requires. 

The  size,  power  and  kind  of  motors  required  will  have 
to  be  worked  out  independently  in  each  case,  and  will 
depend  greatly  on  the  size  of  pumps  and  the  heads  against 
which  they  will  have  to  discharge.  It  might  be  found 
advisable  in  some  cases  to  provide  a  separate  chamber  for 
the  float  which  operates  the  automatic  switch  to  rise  and  fall 


SEWAGE    PURIFICATION    AND    DISPOSAL  173 

in.  When  advisable,  the  chamber  maybe  made  of  masonry, 
wood  or  iron,  but  must  be  open  at  the  bottom  to  admit 
sewage.  The  inlets  are  not  shown  in  the  illustration,  but 
the  sewage  should  be  passed  through  a  screen  of  not  larger 
than  i  %  -inch  mesh,  for  6  to  8-inch  pumps,  before  being 
discharged  into  the  wells,  and  of  not  larger  than  i-inch 
mesh  for  smaller  pumps. 


GASOLINE  PUHPINQ  STATION 

Where  a  large  quantity  of  sewage  must  be  handled 
daily,  the  constant  supervision  of  an  engineer  will  be  nec- 
essary to  look  after  the  machinery,  so  that  the  automatic 
apparatus  can  be  dispensed  with.  Under  such  conditions, 
gasoline  or  kerosene  engines  will  be  found  the  least  expen- 
sive to  operate,  and,  everything  considered,  will  probably 
be  the  most  satisfactory  forms  of  prime  movers. 

The  interior  of  a  gasoline  or  kerosene  engine  pumping 
plant  is  shown  in  perspective  in  Fig.  65.  In  this  plant 
there  are  two  separate  wells  formed  by  a  partition  bviilt  in 
the  center  of  the  large  circular  basin.  In  each  well  is 
located  a  submerged  type  of  centrifugal  pump,  with  a  ver- 
tical shaft  terminating  in  a  bevel  gear  above  the  floor.  The 
two  pumps  are  wholly  independent  of  each  other,  so  that 
one  cannot  be  used  to  pump  sewage  from  the  other  well. 
Further,  each  pump  is  valved  so  that  when  not  in  use,  or 
when  disconnected,  the  discharge  pipe  can  be  put  out  of 
service  by  closing  the  valve.  By  this  means,  either  pump 
can  be  operated  separately,  or  both  pumps  can  be  oper- 
ated at  the  same  time. 

The  inlets  to  the  two  wells  are  valved,  so  that  sewage 
can  be  cut  off  from  either ;  and  before  reaching  the  sluice 
gates,  the  sewage  must  pass  through  a  screen,  to  remove  all 
substances  which  would  clog  or  otherwise  interfere  with 
the  operation  of  the  pumps. 

There  are  two  engines  used  for  operating  the  pumps. 
Nominally,  one  engine  is  intended  to  operate  one  particular 
pump,  but  the  engines  are  so  connected  with  shafting  that 


174 


SEWAGE    PURIFICATION    AND    DISPOSAL 


Fig.  65 

either  engine  can  operate  either  or  both  pumps,  or  the  two 
engines  can  work  in  unison,  giving  a  common  impulse  to 
the  horizontal  driving  shaft. 

Wherever  internal  combustion  engines  are  used,  pro- 
vision must  be  made  to  keep  them  cool.    This  is  usually 


SEWAGE    PURIFICATION    AND    DISPOSAL  175 

accomplished  by  circulating  water  through  a  water-jacket 
casing  around  the  cylinders,  and  passing  the  heated  water 
through  a  radiator  to  cool.  Such  a  method  of  cooling 
necessitates  a  supply  of  cold  water  and  means  for  circulating 
it  through  the  cooling  system.  Batteries  are  required  for 
the  ignition  system,  and  a  good  muffler,  while  not  indis- 
pensable, is  at  all  events  advisable,  as  it  deadens  the  noise 
of  the  explosions  in  the  cylinders.  The  fuel  tank  may  be 
situated  at  any  convenient  point  about  the  premises,  and 
need  not  be  at  a  higher  elevation  than  the  engines,  as  the 
gasoline  is  drawn  into  the  cylinders  by  suction.  If  buried 
in  the  ground,  the  tank  should  first  be  covered  with  a  coat- 
ing of  some  good  preservative,  such  as  asphaltum,  to  pro- 
tect the  metal  from  rusting. 

The  foregoing  illustrations  are  offered  only  as  sugges- 
tions, to  show  the  application  of  electricity  and  internal 
combustion  engines  to  sewage  pumping.  Any  other  type 
of  machinery  may  be  used  for  the  same  purpose,  the  chief 
considerations  being  efficiency  and  economy. 


DISPOSAL  OF  SEWAGE 


DISPOSAL  OF  CRUDE  SEWAGE  BY  DILUTION 

Disposal  into  Tide-water— Cities  situated  on  the  shore 
of  an  ocean  often  solve  the  problem  of  sewage  disposal  by 
discharging  the  crude  sewage  direct  into  tide-water,  de- 
pending on  the  ebb  tide  to  carry  all  solid  matter  out  to  sea, 
while  the  vast  dilution  of  the  large  body  of  water  suffi- 
ciently cares  for  the  matter  in  solution.  This  practice  has 
not  been  objectionable  in  the  past,  when  the  volume  of 
sewage  was  small,  or  when  a  large  quantity  of  sewage  was 
discharged  into  tide  water  in  small  quantities  from  numer- 
ous outfalls,  advantageously  situated  along  the  shore  line. 
As  communities  grow,  however,  the  problem  increases,  and 
it  is  doubtful  if  at  the  present  time  it  is  either  advisable  or 
permissible  for  large  centers  of  population  to  so  dispose  of 
their  sewage.  This  method,  which,  under  favorable  con- 
ditions, may  be  suitable  for  a  city  of  10,000  inhabitants, 
might  be  hopelessly  inadequate  for  a  community  of  half  a 
million  people.  Further,  if  the  outfall  is  situated  in  a  pro- 
tected spot,  or  the  city  is  on  the  shore  of  a  protected  bay, 
continued  discharges  of  sewage  into  such  a  harbor  are  liable 
to  cause  a  nuisance.  In  the  vicinity  of  beaches  used  for 
bathing,  discharging  the  sewage  into  tide-water  would  be 
not  only  a  disagreeable  nuisance,  but  might  prove  a  source 
of  disease,  while  in  the  vicinity  of  oyster  or  clam  beds  such 
a  practice  would  be  attended  with  grave  danger. 

In  the  past,  when  no  adequate  method  of  purifying 
sewage  was  known,  the  practice  of  disposal  into  tide-water 
was  permissible,  and  in  a  manner  solved  the  problem  of 
sewage  disposal.  When  a  large  number  of  people  are 
grouped  together,  an  unusual  or  abnormal  condition  is  cre- 
ated ;  vast  quantities  of  organic  matter  in  the  form  of  food- 
stuffs are  shipped  to  the  community,  and  some  method  of 
disposing  of  the  refuse  from  such  foodstuffs  must  be  devised. 
Discharge  into  tide-water  seemed  a  happy  solution  of  the 
problem  in  former  times,  but  the  large  cities  on  the  sea- 
coast  are » now  turning  their  attention  to  purifying  the 


SEWAGE    PURIFICATION    AND    DISPOSAL  177 

sewage  before  discharging  it  into  the  water;  and  what  is 
found  desirable  for  large  cities  will  apply  likewise,  although 
in  a  less  degree,  to  small  cities  situated  on  the  seashore. 
A  less  degree  of  purification  is  required  for  sewage  which 
is  discharged  into  the  ocean  than  would  be  required  for  an 
effluent  which  is  to  be  discharged  into  a  stream  or  small 
fresh  water  lake,  unless  the  sewage  is  discharged  into  the 
ocean  near  shell  fish  beds,  in  which  case  an  extra  degree  of 
purification  might  be  desirable.  Ordinarily,  however,  if 
the  process  of  purification  has  reduced  the  easily  putresci- 
ble  bodies,  the  more  stable  or  slowly  decomposing  matter 
may  be  discharged  into  tide-water. 

Disposal  into  Fresh  Water  Lakes — The  practice  of 
disposing  of  crude  sewage  into  fresh  water  lakes  is  more 
objectionable  than  disposing  of  it  into  tide-water.  The 
water  from  lakes  is  used  for  water  supplies,  and  the  entire 
water  supply  for  a  community  might  become  infected  from 
sewage  and  cause  an  epidemic  of  disease  in  the  town  sup- 
plied with  the  water,  unless  it  is  filtered  before  being 
delivered  to  the  consumers.  When  sewage  is  discharged 
into  fresh  water,  the  outfall  should  be  at  least  one-half 
mile  from  the  shore,  well  away  from  a  waterworks  intake, 
and  the  sewage  should  be  discharged  from  a  number  of 
outlets  along  the  outfall  sewer,  in  order  to  dilute  the 
sewage  to  a  high  degree  in  the  shortest  possible  time. 

Disposal  into  Streams — It  is  commonly  supposed  that 
flowing  streams  into  which  sewage  has  been  discharged 
have  the  power  of  purifying  themselves,  and  that  a  few 
miles  below  the  point  of  contamination  the  water  will  be 
as  pure  as  before  receiving  the  sewage.  This  power  of 
water  to  cleanse  itself,  which  is  known  as  the  self-purifica- 
tion of  streams,  is  more  limited  than  is  generally  supposed, 
and  depends  for  its  operation  on  the  presence  of  a  suitable 
supply  of  oxygen  and  sufficient  aerobic  bacteria.  So  long 
as  the  dissolved  oxygen  in  a  stream  is  greater  than  25  per 
cent,  of  saturation,  the  nitrifying  bacteria  can  perform 
their  life's  work,  but  should  the  proportion  of  organic 
matter  in  a  stream  be  increased  so  that  the  percentage  of 


178 


SEWAGE    PURIFICATION    AND    DISPOSAL 


saturation  falls  much  below  25,  the  conditions  are  changed, 
and  instead  of  aerobic  decomposition,  anaerobic  putrefac- 
tion takes  place,  foul  smelling  gases  are  produced  and  the 
stream  becomes  an  open  sewer ;  even  when  the  percentage 
of  saturation  falls  below  50,  at  times,  the  water  is  liable  to 
become  offensive.  Evidently,  then,  there  is  a  limit  to  the 
quantity  of  organic  matter  which  can  be  purified  in  a  run- 
ning stream.  This  limit  has  been  variously  estimated  in 
the  case  of  sewage  by  different  aiithorities,  who  allow  a 
certain  number  of  volumes  of  water  to  each  volume  of 
sewage.  These  estimates,  however,  do  not  take  account  of 
the  difference  in  the  strength  of  different  sewages  nor  the 
condition  of  freshness  or  staleness  when  discharged  into 
the  water,  so  that  at  best  they  are  merely  approximations. 
The  proportion  of  sewage  which,  according  to  various 
authorities,  can  be  discharged  into  streams  with  safety  may 
be  found  in  Table  XVII. 

TABLE  XVII — SAFE  DILUTION  OF  SEWAGE  IN  STREAMS 


Authority 

Nuisance 
Probable 

Nuisance 
Improbable 

Pettenkofer     
Stearnn  ...... 

j    1  of  sewage 
"j  to  15  of  water 
j    1  of  sewage 

Herring  
Goodnough      

(    1  of  sewage 
(  to  16  of  water 
j    1  of  sewage 
(  to  23  of  water 

"j  to  40  of  water 
1  of  sewage 
to  45  of  water 
1  of  sewage 
to  36  of  water 

Dilution  of  sewage  is  not  of  itself  a  purification,  but 
merely  an  aid  to  purification.  By  separating  widely  the 
particles  of  organic  matter  in  the  sewage,  dilution  enables 
the  micro-organisms  in  the  water  as  well  as  the  larger 
microscopic  forms  of  animal  organisms,  the  Crustacea, 
rotifers  and  protozoa,  to  carry  on  their  work  of  reduction  in 
the  presence  of  an  abundant  supply  of  oxygen.  It  is  due 
to  these  microscopic  forms  of  life  that  the  self-purification 
of  streams  is  effected.  Self-purification  of  streams  is  not 


SEWAGE    PURIFICATION    AND    DISPOSAL  179 

by  any  means  a  rapid  process,  and  the  distance  below  the 
point  of  pollution  where  the  water  regains  its  original 
purity  is  a  matter  of  doubt.  The  River  Pollution  Commis- 
sion of  Great  Britain  (1874)  concluded  that  sewage  mixed 
with  twenty  times  its  volume  of  water  would  be  only  two- 
thirds  purified  in  flowing  168  miles  at  a  rate  of  i  mile 
an  hour. 

The  water  of  streams  with  a  sluggish  flow  will  be  puri- 
fied in  a  much  shorter  distance  than  will  streams  with  a 
high  velocity.  This  is  due  to  the  fact  that  sedimentation 
is  greater  when  the  flow  of  a  stream  is  at  a  depositing 
velocity  than  when  the  current  is  swift  enough  to  carry 
suspended  matter  along.  The  additional  time  in  which 
the  reducing  organisms  have  to  work  is  an  additional  factor 
in  promoting  clarification.  Sedimentation,  however,  is  not 
a  purification,  and  although  the  lower  reaches  of  a  river 
below  the  point  of  sedimentation  are  purified,  the  deposits 
of  sediment  foul  the  river  bottom,  kill  or  drive  away  fish, 
and  in  course  of  time  create  a  nuisance  in  the  stream. 
Such  is  the  condition  of  the  Passaic  River  in  New  Jersey, 
which  at  present  is  a  large  open  sewer,  so  foul  that  in 
its  water  fish  cannot  live. 


DISPOSAL  OF  SLUDGE 

Composition  of  Sludge — Sewage  sludge  is  the  solid 
residue  which  settles  to  the  bottom  of  septic  and  sedimenta- 
tion tanks  or  other  receptacles  used  in  sewage  purification. 
According  to  the  experiments  of  the  Massachusetts  Board 
of  Health,  sewage  carries  in  suspension  one  part  of  solid 
organic  matter  in  every  thousand  parts  of  water,  and  this 
solid  part  of  organic  matter  unites  with  nine  hundred  and 
ninety-nine  times  its  bulk  to  form  sludge. 

The  composition  of  septic  tank  sludge  differs  from 
chemical  precipitation  sludge  both  in  quantity  and  in 
chemical  composition,  and  the  composition  of  chemical 
precipitation  sludge,  also  septic  tank  sludge,  depends  on  the 


180  SEWAGE    PURIFICATION    AND    DISPOSAL 

composition  of  the  sewage,  the  amount  of  storm  water 
which  enters  the  sewers  and  how  carefully  the  sewage 
is  screened  before  entering  the  tanks.  Ordinarily,  the 
sludge  in  a  septic  tank  consists  of  about  90  per  cent,  water 
and  10  per  cent,  solids,  a  large  percentage  of  the  solids 
being  inorganic  matter. 

Sludge  is  the  most  objectionable  and  at  the  same  time 
the  most  difficult  part  of  sewage  to  dispose  of  without 
creating  a  nuisance  and  the  method  of  disposal  will  depend 
to  a  great  extent  on  the  location  of  the  plant  and  amount  of 
sludge  to  dispose  of. 

Disposal  of  Sludge  in  Deep  Water — When  purification 
works  are  situated  close  to  the  seashore,  the  sludge  from 
tanks  can  be  disposed  of  by  running  it  onto  dumping  scows 
which  are  towed  out  to  deep  water  where  the  sludge  may 
be  dumped.  Whether  it  would  be  good  practice  to  dump 
sludge  in  the  Great  Lakes  is  doubtful,  but  if  the  practice 
should  be  resorted  to,  the  scows  should  be  unloaded  well 
away  from  the  intake  to  a  water  supply  system,  and  a 
sufficient  distance  from  shore  so  as  not  to  foul  the  beaches. 

Burning  of  Sludge — Sludge  filters  of  coarse  material 
may  be  prepared,  and  the  surface  of  the  bed  covered  with 
hay,  straw  or  other  combustible  material,  after  which  the 
sludge  may  be  pumped  or  allowed  to  flow  by  gravity  onto 
the  sludge  beds.  After  the  water  has  seeped  away  and  the 
remaining  moisture  evaporated,  the  sludge  and  straw  or 
hay  can  be  forked  out,  placed  in  a  heap  and  burned.  If 
there  is  no  sludge  bed  at  the  plant  the  sludge  can  be  mixed 
with  combustibles,  such  as  peat,  tanbark  or  sawdust  and 
disposed  of  by  burning.  Sludge  may  also  be  burned  in 
a  garbage  destructor,  as  is  done  in  several  plants  where 
garbage  destructors  and  purification  plants  are  combined. 

Sludge  Used  as  Fertilizer — When  sludge  is  to  be  used 
as  a  fertilizer  it  may  be  run  or  pumped  onto  agricultural 
fields  and  plowed  under.  It  may  be  mixed  with  earth, 
loam,  vegetable  mold,  leaves,  grass,  stable  manure,  ashes, 
sawdust  or  any  other  suitable  material,  and  piled  in  com- 
post heaps  for  future  use;  or  the  sludge  may  be  deposited 


SEWAGE  PURIFICATION  AND  DISPOSAL         isi 

in  large  open  basins,  surrounded  by  an  embankment  and 
left  for  the  moisture  to  evaporate,  after  which  it  can  be 
carted  away  to  use  for  fertilizer  or  to  fill  low  lands.  When 
sludge  is  used  for  filling  low  lands,  each  application  of 
sludge  to  the  soil  should  be  covered  with  a  layer  of  earth, 
ashes  or  like  material.  Ordinary  sludge  from  settling  tanks 
cannot  be  spread  upon  land  without  creating  offensive 
odors,  while  sludge  from  septic  tanks  can  be  disposed  of  in 
that  manner  without  being  offensive.  The  sludge  from  the 
settling  basins  receiving  the  effluent  of  sprinkling  filters 
is  much  less  offensive  than  ordinary  sludge  and  can  be 
applied  to  land  with  practically  no  nuisance. 

Sludge  may  also  be  mixed  with  lime,  then  compressed 
into  cakes  in  filter  presses,  and  in  this  form,  which  makes  it 
easy  to  handle,  conveyed  to  accessible  places  for  fertilizer 
or  for  filling  material.  The  value  of  pressed  sludge,  how- 
ever, seldom  equals  the  cost  of  the  lime  and  pressing. 
While  the  sludge  possesses  no  greater  fertilizing  value  than 
an  equal  weight  of  barnyard  manure,  on  the  other  hand  it 
is  open  to  the  objection  that  it  does  not  give  good  results 
as  a  fertilizer  when  applied  continuously  year  after  year 
to  the  same  piece  of  land. 

Some  sewage  sludges  are  so  rich  in  fats  that  it  is  prac- 
ticable to  recover  the  grease  or  use  it  as  fuel.  The 
separation  of  grease  from  sludge  does  not  deprive  it  of  its 
manurial  value,  as  the  nitrogen  is  not  extracted. 

Disposal  of  Sludge  on  Land — There  are  two  methods 
of  disposing  of  sludge  on  land.  In  the  first  method,  the 
sludge  is  pumped  or  otherwise  discharged  on  to  land  prop- 
erly embanked  to  a  depth  of  8  or  10  inches,  and  the  water 
allowed  to  disappear  by  percolation  and  evaporation, 
leaving  the  dry  residue,  which  may  be  covered  with  a  thin 
layer  of  earth. 

In  the  second  method,  the  ground  is  dug  in  a  series  of 
long  parallel  trenches,  3  feet  wide  by  18  inches  deep,  with 
a  space  of  3  feet  between  trenches,  and  the  excavated 
material  is  piled  up  on  the  spaces  between.  A  large  main 
trench  is  dug  at  right  angles  to  the  sludge  disposal  trenches 


182  SEWAGE    PURIFICATION    AND    DISPOSAL 

and  the  liquid  sludge  is  pumped  into  this  main  trench, 
from  where  it  runs  to  the  branches. 

The  supernatant  water  and  thin  sludge  are  first 
pumped  into  the  trenches,  and  allowed  to  fill  the  lower 
ends,  the  thicker  sludge  being  pumped  in  after  the  greater 
part  of  the  water  has  become  absorbed  by  the  bottom  and 
sides  of  the  excavations.  Small  earthen  ramparts  are  left 
in  the  trenches  at  regular  intervals  of  about  50  feet,  to 
intercept  the  solids  and  allow  the  water  to  run  off  in  front. 

In  about  three  days  after  the  trenches  are  filled,  the 
sludge  is  sufficiently  set  so  a  light  covering  of  screened 
soil  about  i  inch  thick  may  be  spread  on  top.  After  a 
further  interval  of  ten  days,  the  sludge  is  generally 
sufficiently  consolidated  so  that  the  trenches  may  be  filled 
with  the  earth,  which  was  originally  excavated. 

In  the  course  of  a  few  weeks  after  the  .filling  of  the 
sludge  trenches  it  is  generally  practicable  to  excavate 
intermediate  trenches  from  the  solid  ground  which  was 
originally  left  between  the  lines  of  the  first  trenches. 

Once  an  entire  field  has  been  covered  with  sludge  in 
this  manner,  it  is  advisable,  under  the  most  favorable 
climatic  conditions,  to  allow  the  field  to  rest  for  at  least 
two  years  before  re-sludging.  This  will  necessitate  at 
least  three  sludge  fields,  to  be  used  in  rotation,  in  a  three 
year  cycle.  A  soil  of  open  porous  texture  with  low  water- 
table  is  the  best  suited  for  sludge  fields. 

Pressing  Sludge—  Sludge  cannot  be  easily  pressed 
without  the  addition  of  lime  or  some  other  substance  to 
give  it  body  and  at  the  same  time  act  as  a  binder  for  the 
particles  of  sludge.  On  account  of  its  comparative  low 
cost,  value  as  a  fertilizer,  and  the  ease  with  which  it  can  be 
procured,  lime  is  generally  used  for  this  purpose. 

The  caking  qualities  of  sludges  vary  considerably,  in 
fact  more  than  would  be  expected.  For  instance,  septic 
sludge  is  more  difficult  to  press  than  either  chemical  pre- 
cipitation sludge  or  sludge  from  plain  settling  tanks,  and 
at  the  same  time  requires  more  lime  to  form  a  cake,  besides 
necessitating  more  care  in  operating  the  presses. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


183 


Special  presses,  one  of  which,  the  Sh river,  is  shown  in 
Fig.  66,  are  required  for  the  pressing  of  sewage  sludge. 
In  operation,  after  the  sludge  has  been  mixed  with  a  certain 
proportion  of  lime,  it  is  forced  into  the  filter  presses  by 
means  of  pumps  or  compressed  air,  under  a  pressure  of  60 
pounds  per  square  inch.  This  pressure  is  sufficient  to  force 
the  water  through  the  filter  cloths,  while  the  solid  matter 
is  arrested  and  retained  in  the  chambers  of  the  filter  press, 
forming  cakes  anywhere  from  i  inch  to  3  inches  thick, 
according  to  the  method  of  treatment.  When  the  presses 
are  full,  that  is,  when  hard  cakes  are  formed,  the  filtrate 


Inlet 


Fig.  66 

ceases  to  flow  from  the  outlet  cocks,  shown  at  the  bottom 
of  the  cells,  which  indicates  that  it  is  time  to  shut  off  the 
pressure,  open  the  press  and  remove  the  cakes.  After  the 
cakes  have  been  removed,  the  press  is  closed  again,  and  is 
then  ready  for  the  operation  to  be  repeated. 

It  takes  about  45  minutes  to  fill  a  press,  and  about 
15  minutes  to  open,  remove  the  cakes  and  close  the  press 
ready  for  another  operation. 

The  consistency  of  sludge  as  it  is  forced  into  sludge 
presses  is  about  95  per  cent,  liquid  and  5  per  cent,  solid. 
The  pressed  cakes  contain  about  70- per  cent,  moisture. 


184  SEWAGE    PURIFICATION    AND    DISPOSAL 

Filter  presses  are  generally  so  installed  that  a  dump  cart 
can  be  backed  under  them,  or  they  are  placed  over  open- 
ings, or  trap  doors  in  the  floor,  so  that  when  the  presses 
are  opened  the  cakes  can  drop  into  dump  carts  without 
any  handling. 

There  is  but  very  little  odor  to  pressed  sludge. 


DISPOSAL  OF  EFFLUENTS 

The  point  or  place  where  the  effluent  from  a  purifica- 
tion works  is  to  be  discharged,  determines  the  degree  of 
purification  required.  For  instance,  when  the  effluent  is  to 
be  discharged  into  tide  water,  into  the  lower  reaches  of  a 
stream  below  the  intake  to  a  water  supply  system,  or  into 
large  bodies  of  fresh  water  like  the  Great  Lakes,  the 
purification  need  be  carried  only  to  the  point  of  reducing 
the  easily  putrescible  matter,  and  the  more  slowly  oxidized 
particles  may  be  discharged  into  the  water  with  the 
effluent.  When  effluent  is  to  be  discharged  into  good  sized 
bodies  of  fresh  water,  or  into  streams  having  a  dry  weather 
flow  of  more  than  seven  times  the  volume  of  sewage,  the 
effluent  as  a  rule  need  not  be  better  than  that  from  sprink- 
ling filters  and  contact  beds,  after  having  been  subjected 
to  sedimentation  to  remove  the  flocculent  matter  carried  in 
suspension.  This  would  include  the  effluents  from  inter- 
mittent filters,  sewage  farms,  and  well-designed  and 
managed  septic  tanks.  Effluents  which  are  to  be  dis- 
charged into  small  fresh  water  lakes  or  into  streams  having 
a  dry  weather  flow  of  less  than  seven  times  the  volume  of 
sewage,  should  be  able  to  pass  the  fish  test,  and  effluents 
which  are  to  be  discharged  into  dry  ditches  or  open 
channels  leading  to  water  courses  should  be  capable  of 
passing  the  incubation  test. 


DISPOSAL  OF  STORM  WATER 

There  is  no  standard  practice  in  the  United  States  at 
present  for  the  treatment  of  storm  water,  but  in  Great 


SEWAGE    PURIFICATION    AND    DISPOSAL  185 

Britain  there  are  certain  rules  laid  down  by  the  Local  Gov- 
ernment Board,  which  must  be  observed.  These  rules  re- 
quire that  sewage  works,  treating  the  sewage  from  separate 
systems,  shall  be  large  enough  to  care  for  twice  the  dry 
weather  flow;  and  when  treating  sewage  from  combined 
sewer  systems,  three  times  the  dry  weather  flow. 

The  law  further  requires  additional  capacity  to  treat 
storm  water  to  the  extent  of  four  times  the  dry  weather 
flow,  in  the  case  of  separate  sewer  systems,  and  three  times 
the  dry  weather  flow  in  the  case  of  combined  sewer  systems, 
at  a  maximum  rate  of  2,900,000  gallons  per  acre  daily,  so 
that  in  either  case,  or  system  of  sewers,  storm  flows,  up  to 
six  times  the  dry  weather  flow,  must  be  treated.  Any  ex- 
cess of  storm  water  above  this  amount  may  be  discharged 
directly  into  streams  without  treatment. 

Storm  water  is  not  treated  on  the  ordinary  sewage  beds, 
but  special  beds  of  coarse  material  are  usually  provided, 
which  are  operated  continuously  as  long  as  the  storm  lasts, 
and  allowed  long  periods  of  rest  between.  In  order  to  keep 
the  storm  water  beds  in  good  condition,  they  are  sometimes 
treated  with  small  amounts  of  sewage  between  storms,  care 
being  taken  that  they  are  not  clogged  when  needed  for  full 
duty. 

The  period  of  sedimentation  is  much  reduced  during 
storms,  so  that  suspended  matter  is  often  carried  to  the 
storm  beds,  where,  together  with  the  grease  which  is  some- 
times present  in  large  amounts,  it  may  give  rise  to  surface 
clogging.  For  this  reason,  storm  beds  require  to  be  scraped 
at  frequent  intervals,  or  have  the  upper  layers  removed. 


DESIGNING    SEWAGE    PURIFICATION 
PLANTS 


GENERAL  CONSIDERATION 

Sewerage  Systems — Sewage  purification  works  are 
often  designed  for  municipalities  or  communities  where  there 
are  no  existing  sewerage  systems,  but  where  the  sewer- 
age system  and  purification  works  are  to  be  designed  to- 
gether and  with  relation  to  each  other.  Generally,  however, 
the  purification  plant  is  required  to  abate  a  nuisance  caused 
by  the  discharge  of  crude  sewage  from  the  sewage  outfalls, 
and  must  be  designed  to  care  for  the  sewage  under  the  ex- 
isting conditions.  If  the  sewers  are  laid  out  on  the  separate 
system,  so  that  the  sanitary  sewers  receive  the  discharge  of 
only  the  household  wastes,  together  with  what  ground  water 
infilters  into  the  conduits,  the  problem  is  comparatively 
simple,  as  it  necessitates  only  the  collecting  of  the  entire 
sewage  from  the  various  outfalls  into  an  intercepting  main, 
and  extending  the  intercepting  main  to  the  site  of  the  puri- 
fication works.  In  some  cases,  extending  the  intercepting 
main  to  the  site  of  the  purification  plant  might  necessitate 
the  collection  of  sewage  in  sewage  wells  or  sumps  and 
pumping  it  to  the  -purification  works.  This  requirement 
would  be  the  same,  however,  whether  or  not  the  sewerage 
system  were  constructed  on  the  separate  or  combined  system. 
When  the  sewers  of  the  city  are  built  on  the  separate  sys- 
tem, the  purification  works  need  be  made  only  large  enough 
to  care  for  a  volume  of  sewage  equal  to  the  water  supply  of 
the  city  during  the  greatest  consumption,  together  with  the 
infiltering  ground  water,  provision  of  course  being  made  to 
increase  the  plant  with  increase  of  population.  The  volume 
of  sewage  to  be  cared  for  can  be  ascertained  by  gaugings, 
and  no  plant  should  be  constructed  without  first  determin- 
ing, as  nearly  as  possible,  the  exact  flow  of  sewage  and 
ground  water  to  be  cared  for. 

When  the  sewerage  of  a  city  is  on  the  combined  system, 
a  separate  system  of  sanitary  sewers  can  be  installed  for  the 
household  waste,  or,  as  is  more  common  practice,  the  exist- 
ing system  of  sewers  can  be  utilized  for  both  sewage  and 


SEWAGE    PURIFICATION    AND    DISPOSAL  187 

rain  water,  and  provision  made  at  the  works  to  care  for 
part  of  the  water  and  allow  the  balance  to  overflow,  without 
being  purified  after  a  certain  percentage  has  been  cared  for. 
When  this  plan  is  resorted  to,  or  when  it  is  required,  as  it 
is  in  Great  Britain,  sufficient  filter  surface  and  tankage  is 
provided  to  care  for  three  times  the  dry  weather  flow  of 
sewage  on  the  regular  filters,  and  the  excess  of  this  up  to 
six  times  the  dry  weather  flow,  is  treated  on  storm  filters. 
All  storm  water  over  and  above  six  times  the  dry  weather 
flow  is  allowed  to  overflow  to  the  sewer  outfall. 

When  the  sewerage  system  is  to  be  built  in  connection 
with  the  purification  works,  the  entire  plant  can  usually  be 
designed  to  work  better  and  more  economically  than  when 
the  purification  works  are  an  after-consideration  which  must 
be  adapted  to  existing  conditions.  As  a  rule,  when  design- 
ing a  system  of  sewers  in  connection  with  a  purification 
plant,  the  separate  system  will  be  found  the  more  satisfac- 
tory, although  local  conditions  might  be  such  that  the  com- 
bined system  would  be  preferable.  The  system  best  suited 
to  the  conditions  can  be  determined  only  after  a  careful  in- 
vestigation of  all  circumstances  bearing  on  the  subject.  In 
an  arid  region  where  the  rainfall  is  light,  and  the  cost  of 
constructing  separate  storm  sewers  great,  the  better  plan 
would  doubtless  be  to  use  the  combined  system  and  make 
provision  for  treating  the  extra  storm  water,  while  in  a 
rainy  region,  separate  systems  would  probably  be  the  more 
economical.  From  a  strictly  sanitary  point  of  view,  there 
is  an  advantage  in  having  storm  water  treated  at  a  purifica- 
tion works,  as  it  thus  insures  the  purification  of  all  organic 
matter  littering  the  streets  before  a  rain  storm.  On  the 
other  hand,  the  increased  volume  of  sewage  to  be  cared  for 
at  the  works,  and  at  a  time  when  the  beds  are  already  sat- 
urated with  rain  water,  necessitates  the  purchase  of  such  a 
large  tract  of  land  and  the  construction  of  such  vast  works 
as  to  be  burdensome  to  most  communities. 

Quantity  of  Sewage  to  be  Provided  For — With  the 
separate  system  of  sewers,  the  quantity  of  sewage  to  be 
provided  for  will  equal  the  volume  of  water  consumed,  plus 


188  SEWAGE  PURIFICATION  AND  DISPOSAL 

what  ground  water  infilters  into  the  sewers.*  The  volume 
of  ground  water  will  depend  to  a  great  extent  on  the  tight- 
ness of  the  joints  in  the  sewer  pipe,  tfte  porosity  of  the 
soil  and  the  height  of  the  water  table.  The  only  safe  prac- 
tice for  determining  the  volume  of  sewage  to  be  provided 
for  is  to  gauge  the  flow  of  sewage  through  the  main  sewer, 
during  wet  weather  and  dry  weather,  at  various  hours  dur- 
ing the  twenty-four,  and  if  possible,  during  the  four  seasons 
of  the  year,  to  determine  the  fluctuation  in  volume  and  the 
maximum  flow.  For  country  institutions,  such  as  asylums, 
and  hotels,  where  the  sewage  purification  plant  must  be 
built  at  the  time  the  buildings  are  erected,  there  are  no 
means  for  determining  the  volume  of  sewage  by  gaugings, 
and  in  such  cases  an  allowance  of  100  gallons  per  capita  per 
day  will  be  found  safe.  Institutions  of  like  character,  now 
operating,  use  approximately  100  gallons  of  water  per  day 
per  capita,  and  it  is  not  likely  that  the  consumption  of  water 
in  other  institutions  to  be  built  would  exceed  that  limit. 
In  designing  the  plant,  however,  it  should  be  made  large 
enough  to  care  for  the  sewage  from  the  largest  number  of 
inmates  the  buildings  can  accommodate. 

In  large  cities  where  the  houses  are  built  closely 
together,  and  where  the  streets  are  paved  so  that  all  rain 
water  from  roofs,  courts  and  streets  will  be  carried  off  in 
storm  sewers,  there  will  be  no  surplus  water  to  drain  into 
the  sewers  and  in  such  cases  the  volume  of  sewage  will 
about  equal  the  consumption  of  water. 

In  Table  XVIII  will  be  found  the  per  capita  daily  con- 
sumption of  water  in  the  fifty  largest  cities  in  the  United 
States  in  1890  and  1900,  arranged  in  order  of  population. 

In  cities  where  there  are  separate  systems  of  sewers, 
rain  leaders  from  the  roofs  of  buildings  are  sometimes 
connected  to  the  house  drain.  When  this  practice  is  fol- 
lowed, it  adds  considerably  to  the  volume  of  sewage  during 
rainstorms  and  should  be  provided  for. 

*  At  Grinnell,  Iowa,  the  flow  of  sewage  in  wet  weather  is  from  three  to  four 
times  the  volume  of  water  pumped  from  the  city  wells.  No  permanent  water  level, 
steepage  at  depths  varying  from  10  to  40  feet. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


189 


In  cities  having  the  separate  system  of  sewers,  in  which 
the  pipes  are  well  joined  with  good  tight  joints,  in  the  ab- 
sence of  exact  data  it  may  be  assumed  that  the  infiltration 
of  ground  water  amounts  to  15  per  cent.  If,  however, 
the  sewers  are  to  be  laid  below  the  level  of  the  water-table, 
pass  through  swampy  ground,  or  follow  the  course  of  old 
covered  streams,  a  greater  allowance,  perhaps  25  per  cent., 
should  be  made. 

A  condition  which  it  is  as  necessary  to  consider  as 
the  per  capita  consumption  of  water,  is  the  fluctuation  of 

TABLE   XVIII 

Per  capita  daily  water  consumption  in  the  fifty  largest  cities  of  the 
United  States  in  1890  and  in  1900,  arranged  in  order  of  population.* 


Increase  or 

Per  Capita 

Decrease  in 

Consumption 
in  Gallons 

Consumption 
in  Ten  Years 

Oitics 

in  Gallons 

1890 

1900 

Incr. 

Deer. 

1.  fNew  York     

79 

116 

37 

2.  Chicago          ..... 

140 

190 

50 

3.  Philadelphia  

132 

229 

97 

4.  f  Brooklyn       ..... 
5.  St.  Louis        ..... 

72 
72 

159 

"87" 

6.  Boston             

80 

143 

63 

.... 

7.  Baltimore       

94 

97 

3 

8.  San  Francisco        .... 

61 

73 

12 

9.  Cincinnati       

112 

121 

7 

10.  Cleveland       

103 

159 

56 

11.  Buffalo            

186 

233 

47 

12.  New  Orleans          .... 

$87 

J48 

11 

13.  Pittsburgh      . 

144 

231 

87 

14.  Washington    

158 

185 

27 

15.  Detroit            

161 

146 

15 

16.  Milwaukee      

110 

80 

30 

17.  Newark           

76 

94 

'l8' 

18.  Minneapolis   .... 

75 

93 

18 

19.  Jersey  City     .         .         .         . 
20.  Louisville       

97 
74 

160 
100 

63 
26 

21.  Omaha            

94 

176 

82 

22.  Rochester       

66 

83 

17 

23.  St.  Paul           

60 

67 

7 

*The  classification  is  by  the  census  of  1890,  so  as  to  include  all  the  cities  in 
the  earlier  grouping. 

tNew  York  and  Brooklyn  consolidated  since  1890. 
$  Only  a  small  part  of  the  population  supplied. 


190  SEWAGE    PURIFICATION    AND    DISPOSAL 

TABLE  XVIII— Continued 


Increase  or 

Per  Capita 

Decrease  in 

f*itip»c 

Consumption 
in  Gallons 

Consumption 
in  Ten  Years 

V_/1L1CS 

in  Gallons 

1890 

1900 

Incr. 

Deer. 

24.  Kansas  City  

71 

62 

9 

25.   Providence     ..... 

48 

54 

"e" 

26.  Denver           

300 

27.  Indianapolis  ..... 
28.  Allegheny      

71 
230 

79 

8 

29.  Albany    "        

19i' 

.... 

30.  Columbus       

'78' 

230 

152 

31.  Syracuse         

68 

102 

34 

32.  Worcester       

59 

70 

11 

33.  Toledo            

72 

119 

47 

34.  Richmond      .    .    . 

167 

100 

'ei' 

35.  New  Haven    ..... 

135 

150 

15 

36.  Paterson         

128 

129 

1 

37.  Lowell            

66 

85 

19 

38.  Nashville        

146 

140 

6 

39.  Scranton         

40.  Fall  River      

29 

36 

7 

41.  Cambridge     

64 

79 

15 

42.  Atlanta           

36 

84 

48 

43.  Memphis         

124 

125 

1 

44.  Wilmington    

113 

90 

.... 

23 

45.  Dayton            

47 

62 

15 

46.  Troy                

125 

183 

58 

.... 

47.  Grand  Rapids        .... 

156 

.  .  .. 

48.  Reading          

"75" 

92 

'ir 

49.  Camden           

131 

280 

149 

50.  Trenton          

62 

99.9 

37.9 

population.  In  some  municipalities,  for  instance  Coney 
Island,  Atlantic  City  and  Saratoga  Springs,  the  winter 
population  is  only  a  fraction  of  what  the  visiting  crowds 
swell  the  summer  population  to.  When  designing  a  puri- 
fication plant  for  such  a  city  the  works  should  be  built  suffi- 
ciently elastic  to  care  for  the  permanent  population  during 
the  whole  year,  the  maximum  population  during  the  sum- 
mer, or  any  other  number  between  these  extreme  limits. 
This  may  be  accomplished  by  means  of  automatic  or  hand- 
operated  weirs  which  may  be  raised  or  lowered,  thus 
increasing  or  decreasing  the  capacity  of  the  tanks,  or 
which  perhaps  is  the  better  method,  by  building  the  plants 


SEWAGE    PURIFICATION    AND    DISPOSAL 


191 


in  a  series  of  units,  any  one  of  which  can  be  put  in  or 
thrown  out  of  service  as  occasion  demands. 

In  designing  a  system  of  sewers  or  a  purification  plant, 
the  probable  increase  in  population  of  the  community  is 
taken  into  consideration  and  provision  made  for  the 
increase. 

Law  for  Increase  of  Population — With  very  few  excep- 
tions, cities  increase  in  population,  and  when  designing  a 
purification  plant  in  order  to  provide  for  the  increased 
volume  of  sewage,  due  to  the  increased  population,  it 
becomes  necessary  to  determine  at  what  rate  the  population 
will  increase.  If  any  unusual  condition  affects  the  city  at 
the  time  the  sewage  purification  works  are  projected,  that 
is,  if  unusual  opportunity  for  engaging  in  business,  a  boom 
in  real  estate,  or  anything  that  is  likely  to  attract  large 
numbers  of  people  to  that  locality,  the  problem  of  deter- 
mining what  the  population  of  the  cities  or  villages  will 
be  at  a  future  date  becomes  a  matter  more  of  conjecture 
than  of  fact.  However,  the  population  of  cities  which 
are  experiencing  only  the  normal  growth  of  ordinary 
American  cities  can  be  fairly  approximately  forecasted. 
This  can  be  done  by  constructing  a  ruled  chart,  and 
plotting  on  the  chart  the  population  for  several  past 
decades.  For  instance,  take  the  population  of  the  two 
cities,  Lancaster,  Pa. ,  and  Maiden,  Mass.  ;  according  to  the 
census  between  1800  and  1890,  they  had  the  population 
shown  in  Table  XIX. 

TABLE  XIX — POPULATION  OF  LANCASTER,  PA.,  AND  MALDEN,  MASS. 


Year 

City 

1800 

1810 

1820 

1840 

1850 

1860 

1870- 

1880 

1890 

Lancaster,  Pa. 

4,292 

5,405 

7,704 

8,417 

12,369 

17,603 

20,233 

25,769 

32,011 

Maiden,  Mass. 

1,059 

1,884 

1,731 

2,010 

2,514 

3,520 

7,370 

12,017 

23,031 

If  these  dates  and  populations  be  now   plotted  on   a 
chart,  as  shown  in  Fig.  67,  the  diagram  will  show  when  the 


192 


SEWAGE    PURIFICATION    AND    DISPOSAL 


material  increase  in  population  commenced,  and  the 
uniformity  or  variation,  as  the  case  may  be,  between  one 
decade  and  another.  In  the  case  of  Lancaster,  Pa.,  it  will 
be  seen  that  a  marked  increase  in  population  commenced 
in  1840,  and  that  there  was  an  almost  uniform  increase  of 
about  600  people  per  annum  for  the  next  fifty  years.  In 
that  case,  it  would  be  safe  to  assume  that  the  increase  in 
population  would  continue  at  that  rate  for  each  10,000 


40,000 
Sfi.OOO 
32,000 
28,000 
24,000 
20,000 
16,000 
12,000 
8  000 

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7 

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JOO          1810          1820          1830          1840          1850          1860          1870          1880          1890          1900          1910          1920          1930 

Fig.  67 

inhabitants,  and  the  population  of  the  city  at  the  end  of 
twenty,  thirty  or  forty  years  could  be  approximately  fore- 
told, providing  no  unusual  conditions  arose,  either  to 
increase  or  decrease  the  rate. 

The  growth  of  Maiden,  Mass.,  was  much  slower  than 
that  of  Lancaster,  but  during  the  period  from  1880  to  1890 
the  population  of  Maiden  increased  approximately  11,000, 
as  against  about  6,500  for  Lancaster.  This  establishes  for 
Maiden  a  rate  of  over  1,000  per  year,  which  would  probably 


SEWAGE    PURIFICATION    AND    DISPOSAL 


193 


represent  the  average  increase  for  that  city.  Of  course, 
some  local  cause,  like  the  establishing"  or  opening  of  mills 
or  factories,  might  account  for  the  increase  in  population  in 
Maiden  during  the  decade  from  1880  to  1890.  In  that  case, 
that  part  of  the  population  which  was  brought  to  the  city 
by  the  unusual  condition  should  be  deducted  from  the 
increase  in  population  for  that  period  and  the  average 
increase  then  found  for  the  twenty  years  from  1870  to  1890. 
In  the  absence  of  suitable  data,  it  is  sometimes  assumed 
that  American  -cities  of  less  than  50,000  inhabitants 
increase  in  population  at  the  rate  of  50  per  cent,  in  from 
eight  to  ten  years,  and  that  in  cities  of  over  50,000  in- 
habitants the  rate  of  increase  is  50  per  cent,  in  from 
sixteen  to  twenty  years.  This  assumption,  however,  is  of 
but  little  real  value,  as  a  reference  to  Table  XX  will  show. 

TABLE  XX — POPULATION  OF  A  NUMBER  OF  THE  SMALLER  CITIES  AND 

TOWNS  OF  THE  UNITED  STATES  AT  EACH  TEN  YEAR 

PERIOD  FROM  1800  TO  1900 


Name  of  Cities 

1800 

1810 

1820 

1830 

1840 

1850 

1860 

1870 

1880 

1890 

Alexandria,  Va.  .  . 
Akron,  Ohio 

4,971 

7,227 

8,218 

8,241 

8,459 

8,784 
3266 

12,652 
3  477 

18,570 
10  006 

13,659 
16  512 

14,839 

27  600 

Auburn  N  Y 

9548 

10  986 

17225 

21  924 

25  858 

Augusta,  Ga 

6,403 

10,217 

12,493 

15  389 

21  891 

33  300 

Bay  City  Mich 

1  538 

7  064 

20  693 

27  839 

Burlington,  Vt.  .  . 

815 

1,690 

2,iii 

3,525 

4,271 

7,585 

7,713 
8  325 

14,887 
12  692 

11,365 
17  317 

14,590 
35  005 

Chelsea,  Mass 

2,390 

6,701 

13395 

18  547 

21*882 

27  909 

Chester,  Pa  
Cohoes  N  Y 

957 

1,056 

657 

817 

1,790 

1,667 
4,229 

4,631 
8800 

9,485 
15  351 

14,997 
19  416 

20,226 
22  509 

Dallas,  Texas  .  .  . 

10,358 

38067 

Dover,  N.  H.  ... 
Danbury,  Conn.  .  . 
Fitchburg,  Mass.  .  . 
Hamilton,  Ohio 

2,062 
3,180 
1,390 

2,228 
3,606 
1,566 

2,871 
3,873 
1,736 

3,449 
4,331 
2,169 

6,458 
4,504 
2,604 

8,196 
5,964 
5,120 
3,210 

8,502 
7,234 
7,805 
7,233 

9,294 
8,758 
11,260 
11,081 

11,687 
11,666 
12,429 
12,122 

12,790 
16,552 
22,037 
17,565 

Jacksonville  Fla 

1,045 

2  118 

6  912 

7  650 

17  201 

Lancaster,  Pa.  .  .  . 
Maiden,  Mass.  .  .  . 
Manchester,  N.  H.  . 
Norristown,  Pa.  .  . 
Newport,  Ky.  .  .  . 
Steubenville,  Ohio  . 
San  Antonio,  Tex. 

4,292 
1,059 

'ioe 

5,405 
1,384 

'413 

6,633 
1,731 
761 

827 

2,539 

7,704 
2,010 
877 
1,089 
715 
2,937 

8,417 
2,514 
3,235 
2,937 

4',247 

12,369 
3,520 
13,932 
6,024 
5,895 
6,140 
3,488 

17,603 
5,865 
20,017 
8,848 
10,046 
6,154 
8,235 

20,233 
7,370 
23,536 
10,753 
15,087 
8,107 
12,256 

25,769 
12,017 
32,630 
13,063 
20,433 
12,093 
20,550 

32,011 
23,031 
44,126 
19,791 
24,918 
13,394 
37,673 

Wilkesbarre,  Pa.  .  . 
Williamsport,  Pa. 

835 
131 

1,225 
334 

755 
624 

2,232 

1,718 
1,353 

2,728 
1,615 

4,253 
5,664 

10,174 
16,030 

23,339 
18,934 

37,718 
27,132 

Sewage  Disposal  in  the  United  States— Raftor  and  Baker 

Location  for  Purification  Works — The  best  location  for  a 
purification  works  must  be  determined  in  each  case  after  a 
careful  examination  of  all  available  sites,  and  weighing  the 


194  SEWAGE    PURIFICATION    AND    DISPOSAL 

conditions  in  favor  of  each.  When  possible,  it  is  well  to 
locate  the  works  within  reasonable  distance  of  the  city,  but 
if  the  cost  of  suitable  property  near  the  city  outweighs  the 
cost  of  pumping,  it  might  be  advisable  to  locate  the  plant 
at  a  considerable  distance  from  the  city,  where  sufficient 
land  can  be  secured  to  provide  for  future  growth  of  the 
city.  The  site  for  the  purification  works  must  be  selected 
with  a  view  to  the  final  disposal  of  the  effluent  with  the 
least  trouble  and  expense.  For  this  reason,  if  several  sites 
offered  about  equal  advantages,  but  one  was  so  situated,  as 
for  instance  on  the  lower  reaches  of  a  river  below  other 
settlements,  that  the  effluent  could  be  discharged  into  the 
water  with  a  less  degree  of  purification  than  at  the  other 
sites,  the  former  location  no  doubt  would  be  the  best. 

As  a  rule,  the  best  location  cannot  be  determined  with- 
out a  careful  estimate  of  the  cost  of  installation  and  main- 
tenance of  a  plant  built  on  the  several  sites. 

Manufacturing  Wastes — When  the  purification  plant  is 
to  be  designed  for  a  manufacturing  city  where  wastes  from 
various  manufacturing  industries  are  discharged  into  the 
sewers,  the  problem  of  whether  to  require  the  various  in- 
dustrial wastes  to  be  partly  purified  before  discharging  them 
into  the  sewers,  or,  assume  that  task  as  a  community  and 
design  the  plant  accordingly,  must  be  considered.  Some- 
times, at  small  cost,  a  manufacturing  concern  can  so  treat 
its  sewage  that  it  can  safely  be  discharged  into  the  sewers. 
In  such  cases,  it  is  but  reasonable  to  expect  them  to  do  so. 
In  other  cases  the  cost  would  be  excessive,  whereas  the 
crude  sewage,  if  discharged  into  the  sewers,  would  become 
diluted  and  add  but  little  to  the  cost  of  operating  the  muni- 
cipal plant.  In  such  cases,  it  would  seem  the  part  of  wis- 
dom to  permit  the  discharge  of  crude  sewage  into  the  sewers. 
The  only  requirement  in  this  regard  when  designing  a 
plant,  is  that  the  policy  to  be  provided  for  will  be  known  to 
the  engineer. 

Degree  of  Purification  Required — The  degree  of  purifi- 
cation required  will  depend  to  a  great  extent  on  the  place 
of  disposal.  It  is  seldom  that  an  effluent  of  great  purity  is 


SEWAGE    PURIFICATION    AND    DISPOSAL  195 

required,  but  simply  one  which  can  be  discharged  under 
the  existing  conditions  without  creating  a  nuisance.  Such 
a  requirement  is  complied  with,  even  when  the  effluent 
contains  considerable  quantities  of  organic  matter,  provided 
the  organic  matter  is  in  a  fairly  stable  condition,  so  that  it 
will  not  rapidly  undergo  putrefactive  decomposition,  or,  if 
a  sufficient  amount  of  reserve  oxygen  is  in  the  effluent  or 
the  water  into  which  it  is  discharged,  to  unite  with  all 
readily  oxidizable  organic  matter  and  thus  prevent  the 
development  of  anaerobic  conditions. 

Deciding  on  System  of  Purification — The  system  of 
sewage  purification  for  any  locality  will  generally  be  deter- 
mined by  the  availability  of  the  several  materials  in  that 
locality,  and  other  local  conditions;  while  throughout  the 
United  States  the  practice  will  vary  widely  in  different 
localities,  within  the  various  sections  the  practice  will  be 
found  quite  similar.  For  instance,  in  the  New  England 
States  which  are  covered  with  a  mantle  of  glacial  drift  that 
contains  sand  suitable  for  intermittent  filtration,1  this 
method  of  purification  no  doubt  will  be  extensively  used.  In 
the  middle  west,  throughout  the  Mississippi  River  and  Ohio 
River  valleys,  where  suitable  sand  is  more  scarce,  the  sep- 
tic tank  and  contact  beds  or  sprinkling  filters  probably  will 
find  their  greatest  application ;  while  in  the  arid  regions  of 
the  far  west  where  sewage  is  valuable  for  irrigation  pur- 
poses, and  the  sparsely  settled  country  offers  suitable  acres 
of  land  for  farming,  sewage  irrigation  will  be  the  most 
suitable. 

No  hard  and  fast  rules  can  be  laid  down  to  determine 
the  type  of  disposal  works  to  use  under  different  conditions. 
The  only  way  to  decide  is  for  the  designer  to  familiarize 
himself  with  the  various  methods  of  purifying  sewage,  to- 
gether with  their  advantages  and  limitations,  and  from  the 
fullness  of  his  knowledge  and  originality  as  a  designer, 
after  obtaining  the  necessary  experimental  data,  decide 
upon  the  method  for  each  case. 

When  Septic  Tanks  are  Advisable — It  is  not  an  easy 
matter  to  state  exactly  when  a  septic  tank  is  advisable.  In 


196  SEWAGE    PURIFICATION    AND    DISPOSAL 

the  New  England  States,  where  suitable  areas  of  sand  of  the 
right  quality  are  available  for  intermittent  filtration,  septic 
tanks  may  be  dispensed  with  and  the  crude  sewage  dis- 
charged directly  onto  the  sand  beds.  If,  however,  the  sand 
areas  are  limited  so  that  higher  rates  of  purification  must 
be  obtained,  the  septic  tank  will  be  found  valuable  for  a 
preliminary  treatment  of  the  sewage.  The  septic  tank  will 
likewise  be  found  valuable  as  a  primary  treatment  in  small 
towns  or  institutions  which  treat  very  fresh  sewage,  as  the 
treatment  breaks  up  the  masses  of  fecal  matter  and  other 
solids.  It  will  also  be  found  valuable  in  cities  where  there 
is  a  considerable  quantity  of  manufacturing  waste,  as  the 
tanks  equalize  the  composition  and  the  flow  of  sewage. 

Beautifying  Sewage  Purification  Plants — For  senti- 
mental, rather  than  for  sanitary  or  economic  reasons,  a  sew- 
age purification  plant  should  be  made  as  attractive  as  pos- 
sible ;  a  judicious  planting  of  trees  and  shrubbery,  sodding 
of  slopes  and  embankments,  and  painting  of  buildings  so  as 
to  make  the  surroundings  attractive,  will  go  far  toward 
overcoming  the  prejudice  existing  against  the  location  of 
such  works  in  a  given  neighborhood.  If  properly  managed, 
there  is  but  little  or  no  odor  from  a  purification  plant,  but 
that  little  odor  will  be  exaggerated  in  the  imagination  of 
the  people  if  the  plant  and  surroundings  are  unattractive, 
while  the  odors  will  be  entirely  overlooked  or  attributed  to 
other  causes  if  the  place  is  laid  out  as  a  parkway.  Odori- 
ferous plants,  such  as  lilacs,  will  do  much  toward  disguising 
any  odors  that  do  exist. 


APPENDIX 


SOME  PHYSICAL  PROPERTIES  OF  SANDS  AND  GRAVELS 

WITH  SPECIAL  REFERENCE  TO  THEIR  USE  IN 

FILTRATION 


BY  ALLEN  HAZEN 
Chemist  in  charge  of  Lawrence  Experiment  Station 


The  experiments  at  the  Lawrence  Experiment  Station 
under  the  direction  of  Hiram  F.  Mills,  C.  E.,  have  necessi- 
tated many  investigations  in  regard  to  the  physical  proper- 
ties of  filtering  materials.  The  following  is  a  brief  account 
of  some  of  the  methods  of  analysis  devised  in  the  course  of 
these  investigations,  together  with  the  more  important 
results  obtained. 


METHOD  OF  ANALYSIS 

A  knowledge  of  the  sizes  of  the  sand  grains  forms  the 
basis  of  many  of  the  computations.  This  information  is 
obtained  by  means  of  mechanical  analyses.  The  sand 
sample  is  separated  into  portions  having  grains  of  definite 
sizes,  and  from  the  weight  of  the  several  portions  the  rela- 
tive quantities  of  grains  of  any  size  can  be  computed. 

Collection  of  Samples — In  shipping  and  handling,  sam- 
ples of  sands  are  best  kept  in  their  natural  moist  condition, 
as  there  is  then  no  tendency  to  separation  into  portions  of 
unequal-sized  grains.  Under  no  circumstances  should  dif- 
ferent materials  be  mixed  in  the  same  sample.  If  the 
material  under  examination  is  not  homogeneous,  samples 
of  each  grade  should  be  taken  in  separate  bottles,  with 
proper  notes  in  regard  to  location,  quantity,  etc.  Eight- 
ounce  wide-necked  bottles  are  most  convenient  for  sand 
samples,  but  with  gravels  a  larger  quantity  is  often  re- 
quired. Duplicate  samples  for  comparison  after  obtaining 
the  results  of  analyses  are  often  useful. 


198  SEWAGE    PURIFICATION    AND    DISPOSAL 

Separation  into  Portions  having  Grains  of  Definite  Sizes 

— Three  methods  are  employed  for  particles  of  different 
sizes — hand  picking1  for  the  stones,  sieves  for  the  sands  and 
water  elutriation  for  the  extremely  fine  particles.  Ignition, 
or  determination  of  albuminoid  ammonia,  might  be  added 
for  determining  the  quantity  of  organic  matter,  which,  as 
a  matter  of  convenience,  is  assumed  to  consist  of  particles 
less  than  o.oi  millimeter  in  diameter. 

The  method  of  hand  picking  is  ordinarily  applied  only 
to  particles  which  remain  on  a  sieve  two  meshes  to  an  inch. 
The  stones  of  this  size  are  spread  out  so  that  all  are  in  sight, 
and  a  definite  number  of  the  largest  are  selected  and 
weighed.  The  diameter  is  calculated  from  the  average 
weight  by  the  method  to  be  described,  while  the  percentage 
is  reckoned  from  the  total  weight.  Another  set  of  the 
largest  remaining  stones  is  then  picked  out  and  weighed  as 
before,  and  so  on  until  the  sample  is  exhausted.  With  a 
little  practice  the  eye  enables  one  to  pick  out  the  largest 
stones  quite  accurately. 

With  smaller  particles  this  process  becomes  too  labori- 
ous, on  account  of  the  large  number  of  particles,  and 
sieves  are  therefore  used  instead.  The  sand  for  sifting 
must  be  entirely  free  from  moisture,  and  is  ordinarily  dried 
in  an  oven  at  a  temperature  somewhat  above  the  boiling 
point.  The  quantity  taken  for  analysis  should  rarely 
exceed  100-200  grams.  The  sieves  are  made  from  carefully 
selected  brass-wire  gauze,  having  as  nearly  as  possible 
square  and  even-sized  meshes.  The  frames  are  of  metal, 
fitting  into  each  other  so  that  several  sieves  can  be  used  at 
once  without  loss  of  material.  It  is  a  great  convenience  to 
have  a  mechanical  shaker,  which  will  take  a  series  of  sieves 
and  give  them  a  uniform  and  sufficient  shaking  in  a  short 
time;  but  without  this  good  results  can  be  obtained  by 
hand  shaking.  A  series  which  has  proved  very  satisfactory 
has  sieves  with  approximately  2,  4,  6,  10,  20,  40,  70,  100, 
140  and  200  meshes  to  an  inch;  but  the  exact  numbers  are 
of  no  consequence,  as  the  actual  sizes  of  the  particles  are 
relied  upon  and  not  the  number  of  meshes  to  an  inch. 


SEWAGE    PURIFICATION    AND    DISPOSAL  199 

It  can  be  easily  shown  by  experiment  that  when  a 
mixed  sand  is  shaken  upon  a  sieve  the  smaller  particles 
pass  first,  and  as  the  shaking  is  continued  larger  and  larger 
particles  pass,  until  the  limit  is  reached  when  almost  noth- 
ing will  pass.  The  last  and  largest  particles  passing  are 
collected  and  measured,  and  they  represent  the  separation 
of  that  sieve.  The  size  of  separation  of  a  sieve  bears  a 
tolerably  definite  relation  to  the  size  of  the  mesh,  but  the 
relation  is  not  to  be  depended  upon,  owing  to  the  irregu- 
larities in  the  meshes  and  also  to  the  fact  that  the  finer  sieves 
are  woven  on  a  different  pattern  from  the  coarser  ones,  and 
the  particles  passing  the  finer  sieves  are  somewhat  larger 
in  proportion  to  the  mesh  than  is  the  case  with  coarser 
sieves.  For  these  reasons  the  sizes  of  the  sand  grains  are 
determined  by  actual  measurements  regardless  of  the  size 
of  the  mesh  and  of  the  sieve. 

It  has  not  been  found  practicable  to  extend  the  sieve 
separations  to  particles  below  o.io  millimeter  in  diameter 
(corresponding  to  a  sieve  with  about  200  meshes  to  an 
inch),  and  for  such  particles  elutriation  is  used.  The 
portion  passing  the  finest  sieve  contains  the  greater  part 
of  the  organic  matter  of  the  sample,  with  the  exception  of 
roots  and  other  large  undecomposed  matters,  and  it  is 
usually  best  to  remove  this  organic  matter  by  ignition  at 
the  lowest  possible  heat  before  proceeding  to  the  water 
separations.  The  loss  in  weight  is  regarded  as  organic 
matter,  and  calculated  as  below  o.oi  millimeter  in  diameter. 
In  case  the  mineral  matter  is  decomposed  by  the  necessary 
heat,  the. ignition  must  be  omitted,  and  an  approximate 
equivalent  can  be  obtained  by  multiplying  the  albuminoid 
ammonia  of  the  sample  by  .50*.  In  this  case  it  is  necessary 
to  deduct  an  equivalent  amount  from  the  other  fine 
portions,  as  otherwise  the  analyses  when  expressed  in 
percentages  would  add  up  to  more  than  one  hundred. 

Five  grams  of  the  ignited  fine  particles  are  put  in  a 
beaker  90  millimeters  high,  and  holding  about  230  cubic 

*The  method  of  making  this  determination  was  given  in  the  American  Chemical 
Journal,  Vol.  12,  p.  427. 


200  SEWAGE    PURIFICATION    AND    DISPOSAL 

centimeters.  The  beaker  is  then  nearly  filled  with  distilled 
water  at  a  temperature  of  20  degrees  Centigrade,  and 
thoroughly  mixed  by  blowing  into  it  air  through  a  glass 
tube.  A  larger  quantity  of  sand  than  5  grams  will  not 
settle  uniformly  in  the  quantity  of  water  given,  but  less 
can  be  used  if  desired.  The  rapidity  of  settlement 
depends  upon  the  temperature  of  the  water,  so  that  it  is 
quite  important  that  no  material  variation  in  temperature 
should  occur.  The  mixed  sand  and  water  is  allowed  to 
stand  for  fifteen  seconds,  when  most  of  the  supernatant 
liquid,  carrying  with  it  the  greater  part  of  the  particles 
less  than  0.08  millimeter,  is  rapidly  decanted  into  a  suitable 
vessel,  and  the  remaining  sand  is  again  mixed  with  an 
equal  amount  of  fresh  water,  which  is  again  poured  off 
after  fifteen  seconds,  carrying  with  it  most  of  the  remain- 
ing fine  particles.  This  process  is  once  more  repeated, 
after  which  the  remaining  sand  is  allowed  to  drain,  and  is 
then  dried  and  weighed,  and  calculated  as  above  0.08 
millimeter  in  diameter.  The  finer  decanted  sand  will  have 
sufficiently  settled  in  a  few  minutes,  and  the  coarser  parts 
at  the  bottom  are  washed  back  into  the  beaker  and  treated 
with  water  exactly  as  before,  except  that  one  minute 
interval  is  now  allowed  for  settling.  The  sand  remaining 
is  calculated  as  above  0.04  millimeter,  and  the  portion 
below  0.04  is  estimated  by  difference,  as  its  direct  deter- 
mination is  very  tedious,  and  no  more  accurate  than  the 
estimation  by  difference  when  sufficient  care  is  used. 

Determination  of  the  Sizes  of  the  Sand  Grains — The 
sizes  of  the  sand  grains  can  be  determined  in  either  of  two 
ways,  from  the  weight  of  the  particles  or  from  micrometer 
measurements.  For  convenience  the  size  of  each  particle 
is  considered  to  be  the  diameter  of  a  sphere  of  equal 
volume.  When  the  weight  and  specific  gravity  of  a 
particle  are  known,  the  diameter  can  be  readily  calculated. 
The  volume  of  a  sphere  is  £  n  d3,  and  is  also  equal  to  the 
weight  divided  by  the  specific  gravity.  With  the  Lawrence 
materials  the  specific  gravity  is  uniformly  2.65  within  very 
narrow  limits,  and  we  have  ^-5  =  -J  n  d3.  Solving  for  d  we 


SEWAGE    PURIFICATION    AND    DISPOSAL  201 

obtain  the  formulae  d=  .9  j/w  when  d  is  the  diameter  of  a 
particle  in  millimeters  and  w  its  weight  in  milligrams.  As 
the  average  weight  of  particles,  when  not  too  small,  can 
be  determined  with  precision,  this  method  is  very  accurate, 
and  altogether  the  most  satisfactory  for  particles  above 
o.  10  millimeter  ;  that  is,  for  all  sieve  separations.  For  the 
finer  particles  the  method  is  inapplicable,  on  account  of 
the  vast  number  of  particles  to  be  counted  in  the  smallest 
portion  which  can  be  accurately  weighed,  and  in  these 
cases  the  sizes  are  determined  by  micrometer  measure- 
ments. As  the  sand  grains  are  not  spherical  or  even 
regular  in  shape,  considerable  care  is  required  to  ascertain 
the  true  mean  diameter.  The  most  accurate  method  is  to 
measure  the  long  diameter  and  the  middle  diameter  at 
right  angles  to  it,  as  seen  by  a  microscope.  The  short 
diameter  is  obtained  by  a  micrometer  screw,  focusing  first 
upon  the  glass  upon  which  the  particle  rests  and  then  upon 
the  highest  point  to  be  found.  The  mean  diameter  is  then 
the  cube  root  of  the  product  of  the  three  observed 
diameters.  The  middle  diameter  is  usually  about  equal  to 
the  mean  diameter,  and  can  generally  be  used  for  it, 
avoiding  the  troublesome  measurement  of  the  short 
diameters. 

The  sizes  of  the  separations  of  the  sieves  are  always 
determined  from  the  very  last  sand  which  passes  through 
in  the  course  of  an  analysis,  and  the  results  so  obtained 
are  quite  accurate.  With  the  elutriations  average  samples 
are  inspected,  and  estimates  made  of  the  range  in  size  of 
particles  in  each  portion.  Some  stray  particles  both  above 
and  below  the  normal  sizes  are  usually  present,  and  even 
with  the  greatest  care  the  result  is  only  an  approximation 
to  the  truth  ;  still,  a  series  of  results  made  in  strictly  the 
same  way  should  be  thoroughly  satisfactory,  notwithstand- 
ing possible  moderate  errors  in  the  absolute  sizes. 

Calculation  of  Results — When  a  material  has  been  sepa- 
rated into  portions,  each  of  which  is  accurately  weighed 
and  the  range  in  the  sizes  of  grains  in  each  portion  deter- 
mined, the  weight  of  the  particles  finer  than  each  size  of 


SEWAGE    PURIFICATION    AND    DISPOSAL 


separation  can  be  calculated  and  with  enough  properly 
selected  separations  the  results  can  be  plotted  in  the  form 
of  a  diagram,  and  measurements  of  the  curve  taken  for 
intermediate  points  with  a  fair  degree  of  accuracy.  This 
curve  of  results  may  be  drawn  upon  a  uniform  scale  using 
the  actual  figures  of  sizes  and  of  per  cents,  by  weight,  or 
the  logarithms  of  the  figures  may  be  used  in  one  or  both 
directions.  The  method  of  plotting  is  not  of  vital  impor- 
tance, and  the  method  for  any  set  of  materials  which  gives 
the  most  easily  and  accurately  drawn  curves  is  to  be  pre- 
ferred. In  the  diagram  published  last  year  the  logarithmic 
scale  was  used  in  one  direction,  but  in  many  instances  the 
logarithmic  scale  can  be  used  to  advantage  in  both  direc- 
tions. With  this  method  it  has  been  found  that  the  curve 
is  often  almost  a  straight  line  through  the  lower  and  most 
important  section,  and  very  accurate  results  are  obtained 
even  with  a  smaller  number  of  separations. 

Examples  of  Calculation  of  Results — Following  are  ex- 
amples of  representative  analyses,  showing  the  method  of 
calculation  used  with  the  different  methods  of  separation 
employed  with  various  materials. 

TABLE  XXI— ANALYSIS  OF  A  GRAVEL  BY  HAND  PICKING,  11,870  GRAMS 
TAKEN  FOR  ANALYSIS 


Number 
of  Stones 
in  Portion 
(Largest 
Selected 
Stones) 

Total 
Weight 

Portion 
Grams 

Average 
Weight  of 
Stones 
Milligrams 

Estimated 
Weight  of 
Smallest 
Stones 
Milligrams 

Correspond- 
ing Size 
Millimeters 

Total 
Weight 
of  Stones 
Smaller 
than  this 
Size 

Per  Cent, 
of  Total 
Weight 
Smaller 
than  this 
Size 

11  870 

100 

id  . 

3,320 

332,000 

250,000 

56 

8,550 

12 

10  .     . 

1,930 

193,000 

165,000 

49 

6,620 

56 

10   .     . 

1,380 

138,000 

124,000 

45 

5,240 

44 

20   .     . 

2,200 

110,000 

93,000 

41 

3,040 

26 

20  .     . 

1,520 

76,000 

64,000 

36 

1,520 

13 

20  .     . 

1,000 

50,000 

36,000 

30 

520 

4.4 

20  .     . 

460 

23,000 

10,000 

20 

60 

.5 

10  .     . 

40 

4,000 

2,000 

11 

20 

.2 

Dust    . 

20 



The  weight  of  the  smallest  stones  in  a  portion  given  in 
the  fourth  column  is  estimated  in  general  as  about  half-way 


SEWAGE    PURIFICATION    AND    DISPOSAL 


203 


between  the  average  weight  of  all  the  stones  in  that  por- 
tion and  the  average  weight  of  the  stones  in  the  next  finer 
portion. 

The  final  results  are  shown  by  the  figures  in  full-faced 
type  in  the  last  and  third  from  the  last  columns.     By  plot- 

Millimeters 
80 


60 


40 


20 


10 


2O  30          36 

DIAMETER  IN  MILLIMETERS 
Fig.  68 


45     49 


ting  these  figures  (Fig.  68)  we  find  that  10  per  cent,  of  the 
stones  are  less  than  35  millimeters  in  diameter,  and  60  per 
cent,  are  less  than  51  millimeters.  The  uniformity  coeffi- 
cient, as  described  below,  is  the  ratios  of  these  numbers,  or 
1.46,  while  the  "effective  size"  is  35  millimeters. 


204 


SEWAGE    PURIFICATION    AND    DISPOSAL 


//.  Analysis  of  a  Sand  by  Means  of  Sieves 

A  portion  of  the  sample  was  dried  in  a  porcelain  dish 
in  an  air  bath.  Weight  dry,  110.9  grams.  It  was  put  into 
a  series  of  sieves  in  a  mechanical  shaker,  and  given  one 
hundred  turns  (equal  to  about  seven  hundred  single  shakes). 
The  sieves  were  then  taken  apart,  and  the  portion  passing 
the  finest  sieve  weighed.  After  noting  the  weight,  the 
sand  remaining  on  the  finest  sieve  but  passing  all  the 
coarser  sieves  was  added  to  the  first,  and  again  weighed, 
this  process  being  repeated  until  all  the  sample  was  upon 
the  scale,  weighing  110.7  grams,  showing  a  loss  by  hand- 
ling of  only  o.  2,  grams.  The  figures  were  as  follows : 


Sieve  Marked 

Size  of 
Separation  of 
this  Sieve 
Millimeters 

Quantity 
of  Sand 
Passing 
Grams 

Per  Cent, 
of  Total 
Weight 

190               ... 

.105 

.5 

.5 

140               

.135 

1.3 

1.2 

100               

.182 

4.1 

3.7 

60               

.320 

23.2 

21.0 

40               

.46 

56.7 

51.2 

20               .... 

.93 

89.1 

80.5 

10               

2.04 

104.6 

94.3 

6       

3.90 

110.7 

100.0 

Plotting  the  figures  in  heavy-faced  type,  we  find  from 
the  curve  (Fig.  69)  that  10  and  60  per  cent,  respectively 
are  finer  than  .25  and  .62  millimeter,  and  we  have  for 
effective  size,  as  described  above,  .25,  and  for  uniformity 
coefficient  2.5. 

///.  A  nalysis  of  a  Fine  Material  with  Elutriation 
The  entire  sample,  74  grams,  was  taken  for  analysis. 
The  sieves  used  were  not  the  same  as  those  in  the  previous 
analysis,  and  instead  of  mixing  the  various  portions  on  the 
scale  they  were  separately  weighed.  The  siftings  were  as 
follows : 


Remaining  on  sieve  10,  above  2.2  millimeters 
Remaining  on  sieve  20,  above  .98  millimeters 
Remaining  on  sieve  40,  above  .46  millimeters 
Remaining  on  sieve  70,  above  .24  millimeters 
Remaining  on  sieve  140,  above  .13  millimeters 
Passing  sieve  140,  below  .13  millimeters 


1.5  grams. 
7.0  grams. 

22.0  grams. 
20.2  grams. 

9.2  grams. 

14.1  grams. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


205 


The  14.1  grams  passing  the  140  sieve  were  thoroughly 
mixed,  and  one-third,  4. 7  grams,  taken  for  analysis.  After 
ignition,  just  below  a  red  heat  in  a  radiator,  the  weight  was 
diminished  by  0.47  gram.  The  portion  above  .08  millimeter 
and  between  .04  and  .08  millimeter,  separated  as  described 
above,  weighed  respectively  1.27  and  1.71  grams,  and  the 
portion  below  .04  millimeter  was  estimated  by  difference 


70 


60 


td 

O    30 


20 


10 


.62 


.105       .182  .32  .46 

DIAMETER  IN  MILLIMETERS 

Fig.  69 


.93 


(4  7  _  (0.47  +  1.27  +  1.71))  to  be  1.25  grams.  Multiplying 
these  quantities  by  3,  we  obtain  the  corresponding  quanti- 
ties for  the  entire  sample,  and  the  calculation  of  quantities 
finer  than  the  various  sizes  can  be  found  on  following  page. 
By  plotting  the  heavy-faced  figures,  we  find  (Fig.  70) 
that  10  and  60  per  cent,  are  respectively  finer  than  .055  and 
.46  millimeter,  and  we  have  effective  size  .055  millimeter 
and  uniformity  coefficient  8. 


206 


SEWAGE    PURIFICATION    AND    DISPOSAL 


Size  of  Grain 

Weight 
Grams 

Size  of 
Largest 
Particles 
Millimeters 

Weight 
of  all  the 
Finer 
Particles 
Grams 

Per  Cent, 
by  Weight 
of  all 
Finer 
Particles 

Above    2.20  ... 

1.50 

74.00 

100 

.98-2.20.         .         .         . 

7.00 

2.20 

72.50 

•   98 

.46-  .98  . 

22.00 

.98 

65.50 

89 

.24-  .46  . 

20.20 

.46 

43.50 

60 

.13-  .24  . 

9.20 

.24 

23.30 

32 

.08-  .13  .... 

3.81 

.13 

14.10 

19 

.04-  .08  . 

5.13 

.08 

10.29 

14 

.01-  .04  . 

3.75 

.04 

5.16 

7 

Loss  on  ignition  (assumed 

to  be  less  than  .01  milli- 

meter)       .... 

1.41 

.01 

1.41 

1.9 

THE  EFFECTIVE  SIZE 

As  a  provisional  basis  which  best  agrees  with  the 
known  facts,  the  size  of  grain  where  the  curve  cuts  the 
ten  per  cent,  line  is  considered  to  be  the  "effective  size" 
of  the  material.  This  size  is  such  that  10  per  cent,  of  the 
material  is  of  smaller  grains  and  90  per  cent,  is  of  larger 
grains  than  the  size  given.  The  results  obtained  at  Law- 
rence indicate  that  the  finer  10  per  cent,  have  as  much 
influence  upon  the  action  of  a  material  in  filtration  as  the 
coarser  90  per  cent.  This  is  explained  by  the  fact  that  in 
a  mixed  material  containing  particles  of  various  sizes  the 
water  is  forced  to  go  around  the  larger  particles  and 
through  the  finer  portions  which  occupy  the  intervening 
spaces,  and  so  it  is  this  finest  portion  which  mainly  deter- 
mines the  frictional  resistance,  the  capillary  attraction  and, 
in  fact,  the  action  of  the  sand  in  almost  every  way. 

Another  important  point  in  regard  to  a  material  is  its 
degree  of  uniformity;  whether  the  particles  are  mainly  of 
the  same  size,  or  whether  there  is  a  great  range  in  their 
diameters.  This  is  conveniently  shown  by  the  "uniformity 
coefficient,"  a  term  used  to  designate  the  ratio  of  the  size 
of  grain  which  has  .60  per  cent,  of  the  sample  finer  than 
itself  to  the  size  which  has  10  per  cent,  finer  than  itself. 
These  sizes  are  taken  directly  from  the  curve  of  results. 

It  is  not  probable  that  the  above  data  regarding  a  sand 


SEWAGE    PURIFICATION    AND    DISPOSAL 


207 


include  all  the  important  points  to  be  known,  or  that  fur- 
ther study  will  not  modify  or  change  the  method  of  calcu- 
lation; but  in  the  absence  of  better  methods  their  use 


oo 


80 


70 


60 


£40 

H 


20 


10 


.01.044)8  .13 


.24 


.98 


DIAMETER  IN  MILLIMETERS 


Fig.  70 

allows  extremely  valuable  approximate  calculations,  which 
would  otherwise  be  almost  impossible. 


DETERMINATION  OF  OPEN  SPACE  AND  WATER  BY  VOLUME 

As  it  is  often  necessary  to  make  determinations  of 
open  space  and  water  in  sands,  a  few  notes  in  regard  to 
the  most  suitable  methods  will  be  given. 


208  SEWAGE    PURIFICATION    AND    DISPOSAL 

The  specific  gravity  of  the  solid  particles  is  obtained 
by  putting  a  weighed  quantity  of  the  thoroughly  dry 
material  into  a  narrow-necked  graduated  flask  of  distilled 
water,  taking  great  care  that  no  air  bubbles  are  inclosed, 
and  weighing  the  displaced  water.  Very  accurate  results 
may  be  obtained  in  this  way.  The  specific  gravity  of  the 
material  as  a  whole  is  obtained  by  weighing  a  known 
volume  packed  as  it  is  actually  used,  or  as  nearly  so  as 
possible.  As  the  material  is  usually  moist,  it  should 
either  be  dried  before  weighing  or  else  a  moisture 
determination  made  and  a  correction  applied.  The 
open  space  is  invariably  obtained  by  dividing  the  specific 
gravity  of  the  material  as  a  whole  when  dry  by  the 
specific  gravity  of  the  solid  particles,  and  deducting  the 
quotient  from  i.  The  results  obtained  by  measuring  the 
quantity  of  water  which  can  be  put  into  a  given  volume 
when  introduced  from  below  are  invariably  too  low,  be- 
cause the  water  is  drawn  ahead  by  capillarity,  and  air  bub- 
bles are  enclosed  and  remain,  often  causing  serious  errors. 
A  rough  estimate  of  the  open  space  can  be  made  from  the 
uniformity  coefficient.  Sharp-grained  materials  having 
uniformity  coefficients  below  2  have  nearly  45  per  cent, 
open  space  as  ordinarily  packed;  and  sands  having  coeffi- 
cients below  3,  as  they  occur  in  the  banks  or  artificially 
settled  in  water,  will  usually  have  40  per  cent,  open  space. 
With  more  mixed  materials  the  closeness  of  packing  in- 
creases, until,  with  a  uniformity  coefficient  of  6  to  8,  only 
30  per  cent,  open  space  is  obtained,  and  with  extremely 
high  coefficients  almost  no  open  space  is  left.  With  round- 
grained  water- worn  sands  the  open  space  has  been  observed 
to  be  from  2  to  5  per  cent,  less  than  for  corresponding 
sharp-grained  sands. 

The  quantity  of  water  contained  in  sand  is  obtained 
by  drying  a  weighed  portion  in  the  usual  way.  The  vol- 
ume of  the  water  is  reckoned  by  the  formula  V=s/>.  gr. 
100M_M  when  sp.  gr.  is  the  specific  gravity  of  the  material  as 
a  whole  when  dry  and  M  is  the  per  cent,  of  moisture  by 
weight.  The  difference  between  this  figure  and  the  open 
space  is,  in  general,  the  air  space. 


SEWAGE    PURIFICATION    AND    DISPOSAL  209 

CAPILLARITY 

To  determine  the  capillarity  of  a  sand  it  is  so  placed 
that  it  is  drained  at  a  defined  level,  great  care  being-  taken 
to  secure  a  compact  packing  free  from  stratification. 
Water -is  put  freely  upon  it,  and  after  a  definite  time, 
usually  twenty-four  or  forty-eight  hours,  sand  samples  are 
taken  at  various  levels  and  water  determinations  made  as 
described  above.  The  results  plotted  give  a  curve  of 
' '  water  capacity. "  * 

The  height  to  which  water  will  be  held  to  such  an 
extent  as  to  prevent  the  circulation  of  air  can  be  roughly 
estimated  by  the  formula  //— ~  when  It  is  the  height  in 
millimeters  and  d  the  effective  size  of  sand  grain.  The 
data  from  which  the  constant  given  above  as  1.5  was  calcu- 
lated are  very  inadequate,  and  consequently  the  formula 
may  require  modification  with  more  extended  observations. 

The  height  to  which  water  is  held  by  capillarity  is 
independent  of  temperature. 


DETERMINATION  OF  FRICTIONAL  RESISTANCE 

To  determine  the  frictional  resistance  of  a  material,  a 
cylinder  of  galvanized  iron  of  convenient  size  is  filled  with 
the  material  packed  under  conditions  as  far  as  possible  like 
those  under  which  it  is  to  be  used.  For  water  filtration 
the  material  is  put  loosely  in  position  and  settled  to  a  com- 
pact condition  by  introducing  water  from  below.  Stratifi- 
cation must  be  carefully  avoided.  Water  is  then  passed 
through  at  definite  rates,  keeping  the  material  covered 
with  an  excess  of  water  and  regulating  the  rate  of  flow  by 
the  faucet  at  the  bottom.  The  accompanying  diagram 
(Fig.  71)  represents  a  section  of  the  apparatus  (not  drawn 
to  scale).  The  loss  of  head  between  two  points  at  a  defi- 
nite distance  apart  and  both  well  within  the  material  under 
examination  is  observed  in  glass  tubes  attached  to  pet 
cocks  covered  with  fine  wire  gauze  to  keep  back  the 

*The  results  of  a  number  of  such  experiments  were  given  in  the  annual 
report  for  1891,  page  432. 


210 


SEWAGE    PURIFICATION    AND    DISPOSAL 


material.  By  proceeding  in  this  way  we  eliminate  the  loss 
of  head  in  the  surface  layer  of  sand,  which  is  always  much 
greater  than  for  corresponding  material  below  the  surface, 
and  is  better  studied  by  itself.  The  friction  \\hen  the 
experiment  is  first  started  is  always  high,  because  many 
air  bubbles  are  retained  in  the  sand;  but  if  water  not 


OVERFLOW 


--X 


::*•  >"•"-•  v'v  sAND*Y:V*'-;{''\V' 


LOSS    OF 
HE.AD 


entirely  saturated  with  air  is  applied  continuously  for  some 
days  the  air  bubbles  are  absorbed  and  constant  normal 
results  are  obtained. 

FRICTION  OF  WATER  IN  SANDS  AND  GRAVELS 

The  frictional  resistance  of  sand  to  water  within  cer- 
tain limits  of  size  of  grain  and  rate  of  flow  varies  directly 


SEWAGE    PURIFICATION    AND    DISPOSAL  211 

as  the  rate  and  as  the  depth  of  sand.  This  is  given  by 
Piefke*  as  Darcy's  law.  I  have  found  that  the  friction 
also  varies  with  the  temperature,  being  twice  as  great  at 
the  freezing  point  as  at  summer  heat  both  for  coarse  and 
fine  sands,  and  also  that  with  different  sands  the  resistance 
varies  inversely  as  the  square  of  the  effective  size  of  the 
sand  grain.  It  probably  varies  also  somewhat  with  the 
uniformity  coefficient,  but  no  satisfactory  data  are  at  hand 
upon  that  point. 

Putting  the  available  data  in  the  shape  of  a  formula, 
we  have 


where 

Fis  the  velocity  of  the  water  in  meters  daily  in  a  solid 
column  of  the  same  area  as  that  of  the  sand, 

c  is  a  constant  factor  which  present  experiments  indi- 
cate to  be  approximately  1,000, 

d  is  the  effective  size  of  sand  grain, 

k  is  the  loss  of  head, 

/  is  the  thickness  of  sand  through  which  water  passes, 

t  is  the  temperature  on  the  Centigrade  scale  (tFa^+1Q 

may  be  substituted  for  the  last  term,  if  desired). 
The  data  at  hand  only  justify  the  application  of  this  formula 
to  sands  having  a  uniformity  coefficient  below  5,  and  effec- 
tive size  of  grain  o.  10  to  3.00  millimeters. 

The  quantity  of  water  which  will  filter  through  a  sand 
when  its  pores  are  completely  filled  with  water  and  in  the 
entire  absence  of  clogging,  with  an  active  head  equal  to  the 
depth  of  sand,  and  at  a  temperature  of  10  degrees  Centi- 
grade, forms  an  extremely  convenient  basis  for  calculation, 
and  for  convenience  is  called  the  "maximum  rate,"  as  it  is 
approximately  equal  to  the  greatest  quantity  of  water  which 
can  be  made  to  pass  the  sand  under  ordinary  working  con- 
ditions. Thus  a  sand  with  effective  size,  .20  millimeter, 
has  a  maximum  rate  of  40  meters  per  day;  with  effective 

*Zeitschrift  fur  Hygiene,  Vol.  VII,  page  115. 


212 


SEWAGE    PURIFICATION    AND    DISPOSAL 


size  .30  millimeter,  the  maximum   rate  is  90  meters  per 
day,  etc. 

TABLE  XXII — SHOWING  RATE  AT  WHICH  WATER  WILL  PASS  THROUGH 
DIFFERENT  SANDS,  WITH  VARIOUS  HEADS,  AT  A  TEMPERA- 
TURE OF  10  DEGREES  CENTIGRADE 


Effective  Size  in  Millimeters,  10  Per  Cent.  Finer  than— 

h 

0.10 

0.20 

0.30 

0.40 

0.50 

1.00 

3.00 

1 

Meters 

Meters 

Meters 

Meters 

Meters 

Meters 

Meters 

per  Day 

per  Day 

per  Day 

per  Day 

per  Day 

per  Day 

per  Day 

.001     .     . 

.01 

.04 

.09 

.16 

.25 

1 

9 

.005    .     . 

.05 

.20 

.45 

.80 

1.25 

5 

45 

.010    .     . 

.10 

.40 

.90 

1.60 

2.50 

10 

90 

.050    .     . 

.50 

2. 

4.50 

8. 

12.50 

50 

.100    .     . 

1. 

4. 

9. 

16. 

25. 

100 

.500    .     . 

5. 

20. 

45. 

80. 

125. 

1.000    .     . 

10. 

40. 

90. 

160. 

2.000    .     . 

20. 

80. 

180. 

320. 

The  effect  of  variation  in  the  temperature  is  shown  by 
the  following-  table : 
RELATIVE  QUANTITIES  OF  WATER  PASSING  AT  DIFFERENT  TEMPERATURES 


Degrees,  Centigrade 

0 

5 

10 

15 

20 

25 

30 

Degrees,  Fahrenheit 

32 

41 

50 

59 

68 

77 

86 

Quantity      .... 

.70 

.85 

1.00 

1.15 

1.30 

1.45 

1.60 

For  gravels  with  effective  sizes  above  3  millimeters,  the 
friction  varies  in  such  a  way  as  to  make  the  application  of  a 
general  formula  very  difficult.  As  the  size  increases  be- 
yond this  point,  the  velocity  with  a  given  head  does  not  in- 
crease as  rapidly  as  the  square  of  the  effective  size ;  and 
with  coarse  gravels,  the  velocity  varies  as  the  square  root 
of  the  head  instead  of  directly  with  the  head  as  in  sands. 
The  influence  of  temperature  also  becomes  less  marked  with 
the  coarse  gravels. 

The  available  data  for  materials  above  3  millimeters, 
which  are  far  less  complete  than  could  be  desired,  have  been 
obtained  entirely  from  screened  gravels  with  uniformity 
coefficients  from  1.4  to  2.0,  and  at  a  temperature  of  10  de- 
grees C.,  or  a  little  above.  The  results  obtained  were 


SEWAGE    PURIFICATION    AND    DISPOSAL 


218 


plotted,  making  a  diagram  from  which  the  table  below  has 
been  prepared.  The  figures  given  in  the  table  must  be 
taken  as  provisional,  and  for  use  only  until  more  extended 
results  are  obtained. 

TABLE  XXIII — SHOWING  RATE  AT  WHICH  WATER  WILL  PASS  THROUGH 
DIFFERENT  GRAVELS  WITH  VARIOUS  HEADS 


h 

T 

Effective  Size  in  Millimeters,  10  Per  Cent.  Finer  than— 

3 

5 

8 

10 

15 

Meters 
per  Day 

Meters 
per  Day 

Meters 
per  Day 

Meters 
per  Day 

Meters 
per  Day 

.0005   
.001     

002 

3.5 

7 
14 
27 
41 
54 
67 
98 
127 
185 
280 
495 

10 
21 
40 
77 
112 
142 
173 
238 
300 
400 
560 
930 

20 
41 
78 
150 
207 
252 
300 
378 
467 
615 
885 
1,310 

30 
58 
110 
208 
275 
340 
385 
480 
580 
750 
1,060 
1,550 

50 
100 
190 
350 
450 
530 
610 
760 
890 
1,110 
1,490 

.004     

006 

.008     
010 

.015     
.020     
.030     
.050     
.100     

h 
"7 

Effective  Size  in  Millimeters,  10  Per  Cent.  Finer  than— 

20 

25 

30 

35 

40 

Meters 
per  Day 

Meters 
per  Day 

Meters 
per  Day 

Meters 
per  Day 

Meter 
per  Day 

.0005  
.001     
.002     
.004     
.006     ... 
.008     ... 
.010     ...              . 
.015     ...               . 
020     ... 
.030     ... 
.050     ...               .          . 
.100     

80 
148 
275 
480 
620 
720 
830 
1,030 
1,180 
1,450 

110 
205 
370 
610 
780 
900 
1,030 
1,260 
1,470 

150 
275 
480 
740 
930 
1,090 
1,220 
1,480 

200 
370 
590 
870 
1,090 
1,270 
1,410 

250 
450 
710 
1,000 
1,240 
1,450 

;;;;!' 

In  making  calculations  in  regard  to  underdrains  for 
either  sewage  of  water  filters,  or  in  regard  to  the  move- 
ments of  ground  waters,  there  should  be  no  perceptible 
clogging  of  porous  materials  free  from  stratification  by  a 
clear  ground  water,  and  the  formulae  given  can  be  used 
with  only  a  moderate  factor  of  safety  to  cover  possible 


214 


SEWAGE    PURIFICATION    AND    DISPOSAL 


errors  of  sampling,  analysis,  and  errors  in  the  formulae 
themselves.  In  estimating-  the  actual  capacity  of  a  filter, 
so  many  other  conditions  come  in — the  presence  of  air 
bubbles  and  especially  the  increased  friction  in  the  upper 
layers — that  it  is  impossible  to  calculate  the  practicable 
rate  of  flow  by  formulae,  and  we  can  only  safely  rely  upon 
actual  results  from  known  materials. 

The  analyses  of  the  materials  used  at  Lawrence  have 
been  given  in  previous  reports  of  the  board  in  connection 
with  the  results  obtained  from  them.  The  following 
table  contains  the  result  of  analyses  of  some  other  mate- 
rials, which  may  be  of  general  interest: 

TABLE  XXIV— MECHANICAL  ANALYSES  OF  SANDS. 


Effective 
Size  10  Per 
Cent.  Finer 
than— 
Millimeters 

Uniformity 
Coefficient 

Filter  Tank  No.  1,  Lawrence,  Mass.  .  .  . 
Filter  Tank  No.  9,  Lawrence,  Mass.  .  .  . 
Filter  Tank  No.  2,  Lawrence,  Mass.  .  .  . 
Sewage  filters  Gardner  Mass  . 

.48 
.18 
.08 
10-  24 

2.4 
2.0 
2.0 
6  14 

Sewage  filters  Marlborough  Mass 

12 

3  4 

Sewage  filters,  South  Framingham,  Mass.  . 
Water  filter  Lawrence  Mass  ... 

.35-.  42 
25-.  30 

4.5 
25-45 

Water  filter  Birmingham  Eng  

27 

1  8 

Water  filter,    Southwalk  &  Vauxhall    Co., 
London,  Eng  

.29 

2  0 

Water  filter,  Poughkeepsie,  N.  Y  

.25-.  35 

1.8-  1.9 

The  data  already  collected  clearly  show  that  a  well- 
selected  material  is  essential  to  successful  filtration;  and, 
with  the  method  of  examination  and  calculation  now  pro- 
posed, we  can  decide  with  confidence  many  otherwise 
indefinite  points,  and  thus  avoid  unnecessary  expense  and 
unsatisfactory  results  from  the  use  of  unsuitable  or  poorly 
arranged  materials. 


SEWAGE    PURIFICATION    AND    DISPOSAL 


215 


Index 


A  PAGE 

Absorption  Tiles,  Depth  of 155 

Absorption  Tiles,  Distance  Apart  of  155 

Absorption  Tiles,  Grade  of 155 

Absorption  Tiles,  Size  of 155 

Acidity  or  Alkalinity  of  Sewage... .  25 
Action  in  Septic  Tanks,  Bacterial 

and  Physical 82 

Advisable,  When  Septic  Tanks  are  195 

Aerator,  Example  of 87 

Aerators 87 

Aerators,  Object  of 87 

Aerobic  Bacteria 18 

Aerobics,  Facultative 18 

Aerobics,  Obligate 18 

Air  and  Water  Capacity  of  Sands  .  93 

Albumen,  Decomposition  of 21 

Albuminoid  Ammonia  in  Sewage.  .  28 

Alkalinity  or  Acidity  of  Sewage..   .  25 

Alternating  Siphons,  Plural 58 

Ammonia  in  Sewage,  Albuminoid  .  28 

Ammonia  in  Sewage,  Free.. 28 

Anaerobic  Bacteria 18 

Anaerobies,  Facultative 18 

Analysis,  Interpretation  of  Sewage  24 

Analysis,  Method  of  Sand 197 

Analysis,  Object  of  Sewage 24 

Analysis  of  Fine  Sand  with  Elutri- 

ation 204 

Analysis  of  Gravel  by  Hand  Pick- 
ing    202 

Analysis    of    Sand    by    Means    of 

Sieves 204 

Analysis  of  Sands,  Mechanical 214 

Analysis  of  Sewage 24 

Analysis  of  Sewage,  Chemical 24 

Anchored  to  Masonry,  Sluice  Gates  51 
Antiseptic  Treatment  of  Sewage ...  160 
Apparatus,  Barbour  Rotation  Dos- 
ing   59 

Apparatus,  Dosing 53 

Apparatus  for  Contact  Beds,  Auto- 
matic   127 

Appendix 197 

Application  of  Sub-surface  Irriga- 
tion to  Hillside 153 

Area     Required    for    Sub-surface 

Irrigation 156 

Arrangement  of  Filter  Beds, Surface  96 
Automatic  Apparatus  for  Contact 

Beds 127 

Automatic  Revolving  Distributors  116 

Automatic  Siphon,  Miller 53 

Automatic  Siphon,  Rhoads-Miller. .  57 

Automatic  Traveling  Distributor. .  115 

B 

Bacteria 14 

Bacteria,  Aerobic 18 

Bacteria,  Anaerobic 18 

Bacteria   at    Different    Depths    in 

Filter  Beds 62 

Bacteria,  Classification  of 17 

Bacteria,  Facultative 18 

Bacteria,  Fermentative 19 

Bacteria  in  Sewage 30 

Bacteria,  Pathogenic 19 

Bacteria,  Products  of 19 

Bacteria,  Putrefactive 19 


PAGE 

Bacterial  and  Physical   Action  in 

Septic  Tanks 32 

Barbour    Rotation    Dosing    Appa- 
ratus   59 

Basins,  Filter 66 

Basins,  Settling 118 

Beautifying    Sewage    Purification 

Plants 196 

Bed,  Example  oi  a  Single  Contact  120 

Bed,  Principles  of  the  Contact 119 

Beds,    Automatic    Apparatus    for 

Contact 127 

Beds,  Bacteria  at  Different  Depths 

in  Filter 62 

Beds,  Capacity  of  Contact 125 

Beds,  Care  of  Filter 97 

Beds,  Contact 119 

Beds,  Depth  of  Material  for  Contact  123 

Beds,  Depth  of  Sprinkling  Filter. . .  105 

Beds,  Double  Contact 127 

Beds,    Effect    of  Temperature    on 

Contact 127 

Beds,  Efficiency  of  Contact 126 

Beds,  Filter 74 

Beds,  Liquid  Capacity  of  Contact. .  123 

Beds,  Material  for  Contact 121 

Beds,    Materials     for     Sprinkling 

Filter 103 

Beds,  Operation  of  Contact 124 

Beds,  Permanency    of    Sprinkling 

Filter 119 

Beds,  Size  of  Material  for  Contact..  122 
Beds,  Size  of  Material  for  Sprink- 
ling Filter 104 

Beds,     Surface     Arrangement     of 

Filter 96 

Biological  Examination  of  Sewage  30 

Brick  Underdrains 73 

Burning  of  Sludge 180 


Calculating  the   Effective"  Size  of 

Sand 77 

Calculating  the  Uniformity  Coeffi- 
cient of  Sand 78 

Capacities     and     Sizes     of    Drain 

Pipes,  Table  of 140 

Capacities  of  Centrifugal  Pumps, 

Table  of 170 

Capacity  and  Size  of  Intermittent 

Filters 88 

Capacity  of  Contact  Beds 125 

Capacity  of  Contact  Beds,  Liquid...  123 

Capacity  of  Drains,  Table  of 71 

Capacity  of  Precipitation  Tanks. . .  166 

Capacity  of  Sands,  Air  and  Water . .  93 

Capacity  of  Septic  Tanks 39 

Capacity  of  Siphons 56 

Capacity  of  Sprinkling  Filters 117 

Capillarity  of  Sand 209 

Carbohydrates 6 

Carbohydrates,  Fermentation  of . . .  20 

Care  of  Filter  Beds 97 

Catch  work  System  of  Irrigation. . . .  144 

Cellulose 6 

Cellulose  Fermentation 20 

Centrifugal  Pumps 168 


216 


SEWAGE    PURIFICATION    AND    DISPOSAL 


PAGE 

Centrifugal  Pumps,Table  of  Capac- 
ities of 170 

Characteristics  of  Sewage,  Physical  1 1 

Chemical  Analysis  of  Sewage 24 

Chemical  Precipitation  of  Sewage.  160 
Chemical  Precipitation  Plant,  Ex- 
ample of  a 163 

Chemical  Precipitation,  Principles 

of :. 160 

Chemical  Precipitation  Tanks,  Con- 
tinuous Flow 163 

Chemical  Precipitation  Tanks,  Fill- 

and-Draw 165 

Chemical      Precipitation      Tanks, 

Sludge  in 167 

Chlorine  in  Sewage. .   26 

Cities,  Table    of    Consumption    of 

Water  in 189 

Classification  of  Bacteria 17 

Classification  of  Sands 75 

Classification  of  Soils >29 

Coefficient  of  Sand,  Calculating  the 

Uniformity 78 

Coefficient  of  Sand,  Uniformity 76 

Collection  of  Samples  of  Sand 197 

Colloidal  Matter 9 

Color,    Odor     and     Turbidity     of 

Sewage 24 

Columbus  Sprinkler  Nozzle Ill 

Comparison    of    Covered  and   Un- 
covered Septic  Tanks 35 

Composition  of  Sands,  Mechanical.  93 

Composition  of  Sewage 1 

Composition  of  Sludge 179 

Composition  of  Soils 129 

Concrete,  Sluice  Gates  with  Thim- 
bles Set  in 51 

Conservation  of  Matter 1 

Construction  of  Septic  Tanks 36 

Consumed,  Oxygen 30 

Consumption  of  Water   in    Cities, 

Table  of 189 

Contact  Bed,  Example  of  a  Single..  120 

Contact  Bed,  Principles  of  the 119 

Contact  Beds 119 

Contact    Beds,    Automatic    Appa- 
ratus for 127 

Contact  Beds,  Capacity  of 125 

Contact  Bed  s,  Depth  of  Material  for  123 

Contact  Beds,  Double 127 

Contact  Beds,  Effect  of  Tempera- 
ture on 127 

Contact  Beds,  Efficiency  of 126 

Contact  Beds,  Liquid  Capacity  of . .  123 

Contact  Beds,  Material  for 121 

Contact  Beds,  Operation  of 124 

Contact  Beds,  Size  of  Material  for. .  122 
Continuous  Flow  Chemical  Precipi- 
tation Tanks 163 

Covered     and     Uncovered    Septic 

Tanks 34 

Covered     and     Uncovered    Septic 

Tanks,    Comparison    of 35 

Crops  for  Sewage  Farm 146 

Crude  Sewage,  Disposal  of,  by  Dilu- 
tion   176 

D 

Decanting  Effluents,  Floating  Out- 
let for 166 

Decomposition  of  Albumen 21 

Decomposition  of  Sewage 14 

Deep  Water,  Disposal  of  Sludge  in  180 


PAGE 

Degree  of  Purification  Required.. . .  194 

Denitrification 23 

Depth  of  Absorption  Tiles 155 

Depth  of  Irrigation  Underdrains. . .  139 

Depth  of  Material  for  Contact  Beds  123 

Depth  of  Sprinkling  Filter  Beds 105 

Description  of  a  Septic  Tank 36 

Designing      Sewage      Purification 

Plants 186 

Details  of  Intermittent  Filters 66 

Details,  Septic  Tank 44 

Details,  Sprinkling  Filter 103 

Determination  of    Open    Space  in 

Sand 207 

Determination  of  Organic  Nitrogen 

in  Sewage 28 

Determination    of     Size    of    Sand 

Grains 200 

Determination  of  Solid   Matter  in 

Sewage 25 

Determination  of  Water  in  Sand ...  207 

Detritus 10 

Detritus  Tank 44 

Detritus  Tank  and  Screen  Chamber  47 
Dilution,  Disposal  of  Crude  Sewage 

by 176 

Dilution    of    Sewage    in    Streams, 

Table  of  Safe 178 

Dimensions  and  Shapes  of  Septic 

Tanks 42 

Disposal  of  Crude  Sewage  by  Dilu- 
tion   176 

Disposal  of  Effluents 184 

Disposal  of  Sewage 176 

Disposal    of    Sewage    into    Fresh 

Water  Lakes 177 

Disposal  of  Sewage  into  Streams. , .  177 

Disposal  of  Sewage  into  Tide  Water  176 

Disposal  of  Sludge 179 

Disposal  of  Sludge  in  Deep  Water..  180 

Disposal  of  Sludge  on  Land 181 

Disposal  of  Storm  Water 184 

Disposal  Plant,  Example  of  a  Sub- 
surface   150 

Distance  Apart  of  Absorption  Tiles  155 
Distance  Between  IrrigationUnder- 

drains 139 

Distributing  System  for  Sub-sur- 
face Irrigation 151 

Distributors,  Automatic  Revolving  116 

Distributors,Proportioning  Sewage  84 

Distributors  for  Sprinkling  Filters  106 

Distributors,  Sewage 81 

Distributors,    Size    of    Half-round 

Flumes  for S6 

Distributor,  Traveling  Automatic..  115 
Dose,    Size     and      Frequency     of 

Irrigation 157 

Dosing  Apparatus 53 

Dosing  Apparatus,  Bar  hour  Rota- 
tion   59 

Dosing  Tank 44 

Double  Contact  Beds 127 

Drainage  of  Soils 133 

Drain   Pipes,  Table    of   Sizes  and 

Capacities  of 140 

Drains,  Table  of  Capacity  of 71 

Driving  Sewage  Pumps,  Method  of  170 


Effective  Size  of  Sand 77 

Effective  Size  of  Sand 206 


SEWAGE  PURIFICATION  AND  DISPOSAL 


217 


PAGE 

Effective  Size  of  Sand,  Calculating 

the 77 

Effect  of  Temperature  on  Contact 

Beds 127 

Effect  of  Temperature  on  Intermit- 
tent Filtration 95 

Effect  of  Temperature  on  Sprink- 
ling Filters 119 

Efficiency  of  Contact  Beds 126 

Efficiency  of  Sprinkling  Filters 117 

Effluents 12 

Effluents,  Disposal  of 184 

Effluents,  Floating  Outlet  for  De- 
canting   166 

Effluents,  Tests  for 31 

Electric  Pumping  Plant 171 

Elutriation,  Analysis  of  Fine  Sand 

with 204 

Examinations  of  Sewage,  Biolog- 
ical   30 

Example  of  a  Chemical  Precipita- 
tion Plant 163 

Example  of  a  Single  Contact  Bed. .  120 
Example  of  a  Sub-surface  Disposal 

Plant 150 

Example  of  an  Aerator 87 

Example  of  an  Intermittent  Filter.  64 

Examples  of  Pumping  Plants 171 

Examples  of  Sprinkling  Filters. ...  100 


Facultative  Aerobies 18 

Facultative  Anaerobies 18 

Facultative  Bacteria 18 

Farms,  Crops  for  Sewage 146 

Farms,Table  of  Statistics  of  Sewage  147 

Fermentation,  Cellulose 20 

Fermentation  of  Carbohydrates. ...  20 

Fermentative  Bacteria 19 

Fertilizer,  Sludge  Used  as 180 

Fields,    Size    of    Underdrains    for 

Irrigation 138 

Fields,  System  of  Underdrains  for 

Irrigation 135 

Fill-and-Draw  Chemical  Precipita- 
tion Tanks 165 

Filter  Basins 66 

Filter  Beds 74 

Filter  Beds,  Bacteria  at  Different 

Depths  in 62 

Filter  Beds,  Care  of 97 

Filter  Beds,  Depth  of  Sprinkling.. .  105 
Filter  Beds,  Materials  for  Sprink- 
ling    103 

Filter  Beds,  Permanency  of  Sprink- 
ling   119 

Filter    Beds,  Size  of   Material  for 

Sprinkling 104 

Filter  Beds,  Surface  Arrangement 

of 96 

Filter  Details,  Sprinkling 103 

Filter,  Example  of  an  Intermittent  64 

Filter  Underdrains 68 

Filter  Underdrains,  Systems  of 68 

Filters,  Capacity  of  Sprinkling 117 

Filters,  Details  of  Intermittent 66 

Filters,  Distributors  for  Sprinkling  106 

Filters,  Efficiency  of  Sprinkling 117 

Filters,  Effect  of  Temperature  on 

Sprinkling 119 

Filters,  Odors  from  Sprinkling 118 

Filters,  Period  of    Flow  Through  • 

Sprinkling 106 


PAGE 
Filters,    Size     and     Capacity     of 

Intermittent 88 

Filters,  Sprinkling 97 

Filtering  Materials 74 

Filtration,   Effect  of  Temperature 

on  Intermittent  95 

Filtration  of  Sewage,  Intermittent      61 
Filtration,     Principles     of     Inter- 
mittent       61 

Filtration,  Principles  of  Sprinkling      97 
Fine  Sand,  Analysis  of,  with  Elutri- 
ation      204 

Fish  Test 31 

Fittings  for  Sub-surface  Irrigation    152 

Fixed  Sprinklers 106 

Floating     Outlet     for     Decanting 

Effluents 166 

Flow  of  Sewage,  Variation  in  the..      13 
Flow  Through  Septic  Tanks,  Uni- 
formity of 41 

Flow      Through      Septic      Tanks, 

Velocity  of 40 

Flow  Through  Sprinkling  Filters, 

Period  of 106 

Flumes    for    Distributors,   Size   of 

Half-round 86 

Formula  for  Proportioning  Sewage 

Distributors 85 

Formula  for  Proportioning  Under- 
drains        69 

Free  Ammonia  in  Sewage 28 

Frequency  and  Size  of  Irrigation 

Dose 157 

Fresh    Water    Lakes,   Disposal    of 

Sewage  into 1 77 

Fractional  Resistance  of  Sand 209 

Friction    of    Water    in    Sand    and 

Gravel 210 

Furrow  and  Ridge  Irrigation 141 


Gas  from  Sewage 10 

Gasoline  Pumping  Station 173 

Gate  Operating  Devices,  Sluice 51 

Gate  Valves 52 

Gates  Anchored  to  Masonry,  Sluice  51 

Gates  and  Valves,  Sluice 49 

Gates  Bolted  to  Pipe,  Sluice 51 

Gates,  Shear 49 

Gates,  Sluice 50 

Gates  with  Thimbles  Set  in 

Concrete,  Sluice 51 

Grade  of  Absorption  Tiles 155 

Grades  for  Irrigation  Underdrains  141 
Grains,  Determination  of  Size  of 

Sand 200 

Grains,  Sand,  Separation  of,  into 

Definite  Sizes 198 

Gravel,  Analysis  of,  by  Hand 

Picking  202 

Gravels  and  Sands,  Friction  of 

Water  in 210 

Gravels  and  Sands,  Physical  Prop- 
erties of 197 

Gravels,  Rate  at  Which  Water 

Passes    Through 213 

H 

Half-round  Flumes  for  Distribu- 
tors, Size  of 8G 

Hand  Picking,  Analysis  of  Gravel 

by 202 


218 


SEWAGE    PURIFICATION    AND    DISPOSAL 


PAGE 

Hillside,  Application  of  Sub-surface 
Irrigation  to 153 


Increase  of  Population,  Law  for....  191 

Incubation  Test 81 

Inorganic  Matter 6 

Intermittent  Filters,  Details  of 66 

Intermittent     Filters,     Size      and 

Capacity    of 88 

Intermittent    Filtration,   Effect   of 

Temperature  on 95 

Intermittent  Filtration  of  Sewage..  61 
Intermittent  Filtration,  Principles 

of 61 

Interpretation  of  Sewage  Analysis.  24 
Irrigation     Application     of     Sub- 
surface, to  Hillside 153 

Irrigation,     Area     Required     for 

Sub-surface 156 

Irrigation,  Catch  work  System  of..  144 
Irrigation,    Distributing    Systems 

for  Sub-surface 151 

Irrigation    Fields,  Size  of   Under- 
drains for 138 

Irrigation  Fields,Sy  stems  of  Under- 
drains for 135 

Irrigation,  Fittings  for  Sub-surface  152 
Irrigation     Plants,    Operation     of 

Sub-surface 158 

Irrigation,  Preparing  Soils  for 188 

Irrigation,  Principles  of  Sewage. . .  128 

Irrigation,Principles  of  Sub-surface  146 
Irrigation,    Quantity    of     Sewage 

Required  for 145 

Irrigation,  Ridge  and  Furrow 141 

Irrigation,  Sewage 128 

Irrigation,  Sub-surface 146 

Irrigation  Tiles,  Method  of  Laying 

Sub-surface 152 

Irrigation  Underdrains,  Depth  of. .  189 
Irrigation    Underdrains,    Distance 

Between 139 

Irrigation,  Underdrains   for   Sub- 
surface   158 

Irrigation  Underdrains,  Grades  for  141 

Irrigation  with  Sewage,  Methods  of  141 


Lakes,    Disposal    of   Sewage    into 

Fresh  Water 177 

Land,  Disposal  of  Sludge  on 181 

Law  for  Increase  of  Population 191 

Laying  Sub-surf  ace  Irrigation  Tiles, 

Method  of 152 

Liquid  Capacity  of  Contact  Beds. . .  123 

Location  for  Purification  Works. . .  193 

M 

Manufacturing  Wastes 194 

Masonry,  Sluice  Gates  Anchored  to  51 

Materials,  Filtering 74 

Materials  for  Contact  Beds 121 

Materials  for  Contact  Beds,  Depth 

of 123 

Materials  for  Contact  Beds,  Size  of  122 

Materials  for  Sprinkling  Filter  Beds  103 
Materials  for  Sprinkling  Filter 

Beds,  Sizes  of 104 

Matter,  Colloidal 9 

Matter,  Conservation  of 1 


PAGE 

Matter,  Inorganic 6 

Matter  in  Sewage,  Determination 

of  Solid 25 

Matter  in  Solution 9 

Matter,  Nitrogenous 5 

Matter,  Organic 2 

Matter,  Suspended 8 

Mechanical  Analysis  of  Sands 214 

Mechanical  Composition  of  Sands. .  93 
Mechanically  Operated  Sewage 

Screens 46 

Method  of  Laying  Sub-surface 

Irrigation  Tiles 1 52 

Method  of  Sand  Analysis 197 

Methoas  of  Driving  Sewage  Pumps  170 

Methods  of  Irrigating  witn  Sewage  141 

Methods  of  Sewage  Purification —  32 

Miller  Automatic  Siphon 58 

Moisture  Soil 180 

N 

Nested  Sieves 79 

Nitrates  in  Sewage 29 

Nitrification , 22 

Nitrites  in  Sewage 29 

Nitrogen  in  Sewage,  Determination 

of  Organic 28 

Nitrogenous  Matter 5 

Nozzle,  Columbus  Sprinkler Ill 

Nozzles,  Sprinkler 112 

o 

Object  of  Aerators 87 

Object  of  Sewage  Analysis 24 

Obligate  Aerobics 18 

Odor,    Color     and     Turbidity     of 

Sewage 24 

Odors  from  Sprinkling  Filters 118 

Open   Space  in  Sand,  Determina- 
tion of. 207 

Operating  Devices,  Sluice  Gate 51 

Operation  of  Contact  Beds 124 

Operation  of    Sub-surface  Irriga- 
tion Plants 158 

Organic  Matter 2 

Organic  N  itrogen  in  Sewage,  Deter- 
mination of 28 

Outlet    for     Decanting    Effluents, 

Floating 166 

Oxygen  Consumed 30 


Pathogenic  Bacteria 19 

Perforated  Tile  Underdrains 73 

Permanency    of    Sprinkling  Filter 

Beds 119 

Period  of  Flow  Through  Sprinkling 

Filters 106 

Physical    and  Bacterial  Action  in 

Septic  Tanks 32 

Physical  Characteristics  of  Sewage      11 
Physical   Properties  of  Sands  and 

Gravels 197 

Picking,    Analysis    of    Gravel    by 

Hand 202 

Pipe  for  Underdrains,  Tile 73 

Pipe,  Sluice  Gates  Bolted  to 51 

Pipes,  Table  of  Sizes  and  Capaci- 
ties of  Drain 140 

Plant,  Example  of  a  Chemical  Pre- 
cipitation       163 


SEWAGE    PURIFICATION    AND    DISPOSAL 


219 


PAGE 

Plant,   Example  of  a  Sub-surface 

Disposal 150 

Plants,  Beautifying  Sewage  Purifi- 
cation    196 

Plants,  Designing  Sewage  Purifica- 
tion   186 

Plants,  Electric  Pumping 171 

Plants,  Examples  of  Pumping 171 

Plants,  Operation    of   Sub-surface 

Irrigation 158 

Plants,  Sewage  Pumping 168 

Plural  Alternating  Siphons 58 

Population,  Law  for  Increase  of. . . .  101 

Precipitants 161 

Precipitating    Sewage,    Table     of 

Reagents  for 162 

Precipitation  of  Sewage,  Chemical.  160 
Precipitation   Plant,  Example  of  a 

Chemical 1(53 

Precipitation,  Principles  of  Chemi- 
cal   160 

Precipitation  Tanks,  Capacity  of. . .  166 
Precipitation    Tanks,    Continuous 

Flow  Chemical 163 

Precipitation  Tanks,  Fill-and-Draw 

Chemical 165 

Precipitation     Tanks,     Sludge     in 

Chemical • 167 

Preparing  Soils  for  Irrigation 133 

Pressing  Sludge 182 

Principles  of   Chemical   Precipita- 
tion.   160 

Principles  of    Intermittent   Filtra- 
tion   61 

Principles  of  Septic  Purification  of 

Sewage 32 

Principles  of  Sewage  Irrigation 128 

Principles  of  Sewage  Purification..  1 
Principles  of  Sprinkling  Filtration.  -97 
Principles   of   Sub-surface    Irriga- 
tion   146 

Principles  of  the  Contact  Bed 119 

Products  of  Bacteria 19 

Properties  of  Sands  and  Gravels, 

Physical 197 

Proportioning  Sewage  Distributors  84 
Proportioning  Sprinkler  Systems..  113 
Proportioning  Underdrains,  Form- 
ula for 69 

Pumping  Plant,  Electric 171 

Pumping  Plants,  Examples  of 171 

Pumping  Plants,  Sewage 168 

Pumping  Station,  Gasoline. 173 

Pumps,  Centrifugal 168 

Pumps,  Methods  of  Driving  Sewage  170 
Pumps,  Table  of  Capacities  of  Cen- 
trifugal   170 

Purification,  Deciding  on  System  of  1P5 

Purification,  Methods  of  Sewage. ...  32 

Purification  of  Sewage 1 

Purification  of  Sewage,  Septic 32 

Purification     Plants,     Beautifying 

Sewage 196 

Purification  Plants,  Designing  Sew- 
age   186 

Purification,  Principles  of  Sewage . .  1 

Purification  Required,  Degree  of. . .  194 

Purification  Works,  Location  for. . .  193 

Putrefactive  Bacteria...  19 


Quantity    of   Sewage    Purified   by 
Different  Sizes  of  Sand 92 


PAGE 

Quantity  of  Sewage  Required  for 
Irrigation 145 

Quantity  of  Sewage  to  be  Provided 
for .  187 


Rate     at     which     Water     Passes 

Through  Gravels 213 

Rate  at  which  Water  Passes 

Through  Sands 212 

Rating  and  Size  of  Sieves 79 

Reagents  for  Precipitating  Sewage, 

Table  of 162 

Reduction  of  Sludge  in  Septic 

Tanks 43 

Removable  Sewage  Screens 45 

Resistance  of  Sand,  Frictional 209 

Revolving  Distributors,  Automatic  116 

Revolving  Sprinklers 114 

Rhoads-Miller  Automatic  Siphon . .  57 

Ridge  and  Furrow  Irrigation 141 

Rotation  Dosing  Apparatus,  Bar- 

bour 59 


Safe      Dilution      of      Sewage      in 

Streams,  Table  of 178 

Samples  of  Sand,  Collection  of 197 

Sand 75 

Sand  Analysis,  Method  of 197 

Sand,   Analysis   of,   by    Means   of 

Sieves 204 

Sand,  Analysis  of  Fine,  with  Elu- 

triation 204 

Sand     and     Gravels,    Friction    of 

Water  in 210 

Sand,    Calculating    the    Effective 

Size  of 77 

Sand,  Calculating  the  Uniformity 

Coefficient  of 78 

Sand,  Capillarity  of 209 

Sand,  Collection  of  Samples  of 197 

Sand,     Determination      of     Open 

Space  in 207 

Sand,  Determination  of  Water  in..  207 

Sand,  Effective  Size  of 77 

Sand,  Effective  Size  of 206 

Sand,  Frictional  Resistance  of 209 

Sand     Grains,     Determination     of 

Size  of 200 

Sand    Grains,  Separation    of,  into 

Definite  Sizes 198 

Sand,  Quantity  of  Sewage  Purified 

by  Different  Sizes  of 92 

Sand,  Scales  for  Weighing 81 

Sands,  Air  and  Water  Capacity  of.  93 
Sands  and  Gravels,  Physical  Prop- 
erties of 197 

Sands,  Classification  of 75 

Sands,  Mechanical  Analysis  of 214 

Sands,  Mechanical  Composition  of.  93 
Sands,  Rate  at  which  Water  Passes 

Through 212 

Scales  for  Weighing  Sand 81 

Screen  Chamber  and'Detritus  Tank  47 
Screens,     Mechanically     Operated 

Sewage 46 

Screens,  Removable  Sewage 45 

Screens,  Sewage 45 

Separation  of  Sand  Grains  into  Def- 
inite Sizes 198 

Septic  Purification  of  Sewage 32 


220 


SEWAGE    PURIFICATION    AND    DISPOSAL 


PAGE 

Septic  Tank  Details 44 

Septic  Tank,  The 32 

Septic  Tanks  are  Advisable,  When  195 
Septic  Tanks,  Bacterial  and  Phys- 
ical Action  in 82 

Septic  Tanks,  Capacity  of 39 

Septic  Tanks,  Construction  of 36 

Septic  Tanks,  Covered  and  Uncov- 
ered    34 

Septic  Tanks,  Description  of 36 

Septic  Tanks,  Reduction  of  Sludge 

in 43 

Septic  Tanks,  Shapes  and  Dimen- 
sions of 42 

Septic  Tanks,  Uniformity  of  Flow 

Through 41 

Septic    Tanks,    Velocity    of    Flow 

Through 40 

Settling  Basins 118 

Sewage,  Acidity  or  Alkalinity  of..  25 

Sewage,  Albuminoid  Ammonia  in  28 

Sewage  Analysis,  Interpretation  of  24 

Sewage,  Analysis  of 24 

Sewage  Analysis,  Object  of 24 

Sewage,  Antiseptic  Treatment  of . .  160 

Sewage,  Bacteria  in 80 

Sewage,  Biological  Examinations  of  30 

Sewage,  Chemical  Analysis  of 24 

Sewage,  Chemical  Precipitation  of  160 

Sewage,  Chlorine  in 26 

Sewage,  Color,  Odor  and  Turbidity 

of.. 24 

Sewage,  Composition  of 1 

Sewage,  Decomposition  of 14 

Sewage,  Determination  of  Organic 

Nitrogen  in 28 

Sewage,    Determination    of    Solid 

Matter  in 25 

Sewage,  Disposal  of 176 

Sewage,  Disposal  of  Crude,  by  Di- 
lution    176 

Sewage,  Disposal  of,  into  Streams  177 
Sewage,    Disposal    of,    into    Fresh 

Water  Lakes 177 

Sewage,    Disposal    of,    into    Tide 

Water 176 

Sewage  Distributors 81 

Sewage   Distributors,    Proportion- 
ing    84 

Sewage  Farms,  Crops  for 146 

Sewage  Farms,  Table  of  Statistics 

of 147 

Sewage,  Free  Ammonia  in 28 

Sewage,  Gas  from 10 

Sewage  in  Streams,  Table  of  Safe 

Dilution  of 178 

Sewage,  Intermittent  Filtration  of  61 

Sewage  Irrigation 128 

Sewage  Irrigation,  Principles  of. . .  128 

Sewage,  Methods  of  Irrigating  with  141 

Sewage,  Nitrates  in 29 

Sewage,  Nitrites  in 29 

Sewage,  Physical  Characteristics  of  11 

Sewage  Pumping  Plants 168 

Sewage  Pumps,  Methods  of  Driving  170 

Sewage  Purification,  Methods  of . . .  32 

Sewage,  Purification  of 1 

Sewage  Purification  Plants,  Beauti- 
fying   196 

Sewage    Purification     Plants,    De- 
signing   186 

Sewage  Purification,  Principles  of.  1 
Sewage,  Purified  by  Different  Sizes 

of  Sand,  Quantity  of 92 


PAGE 

Sewage    Required    for    Irrigation, 

Quantity  of 145 

Sewage  Screens 45 

Sewage  Screens,  Mechanically  Op- 
erated  ". 46 

Sewage  Screens,  Removable 45 

Sewage,  Septic  Purification  of 32 

Sewage,  Staleness  of 11 

Sewage,  Strength  of 12 

Sewage,  Table  of  Reagents  for  Pre- 
cipitating   162 

Septic  Tanks,  Comparison  of  Cov- 
ered and  Uncovered 85 

Sewage,  Temperature  of 14 

Sewage,  Temperature  of 25 

Sewage  to  be  Provided  for,  Quan- 
tity of 187 

Sewage,  Variation  in  the  Flow  of..  13 

Sewerage  Systems 1H6 

Shape  and    Dimensions   of   Septic 

Tanks 42 

Shear  Gates 49 

Sieves,  Analysis  of  Sand  by  Means 

of.... . 204 

Sieves,  Nested 79 

Sieves,  Size  and  Rating  of 7'.) 

Sieves,  Standardizing 80 

Single  Contact  Bed,  Example  of  a..  120 

Siphon,  Miller  Automatic —  53 

Siphon,  Rhoads-Miller  Automatic.  57 

Siphons,  Capacity  of 56 

Siphons,  Plural  Alternating 58 

Size  and  Capacity  of  Intermittent 

Filters i 83 

Size   and    Frequency  of  Irrigation 

Dose 157 

Size  and  Rating  of  Sieves ". . .  79 

Size  of  Absorption  Tiles 155 

Size  of  Half-round  Flumes  for  Dis- 
tributors   86 

Size  of  Material  for  Contact  Beds. .  122 
Size    of    Material    for    Sprinkling 

Filter  Beds 104 

Size     of     Sand,    Calculating     the 

Effective 77 

Size  of  Sand,  Effective 77 

Size  of  Sand,  Effective 206 

Size  of  Sand  Grains,  Determination 

of 200 

Size  of  Underdrains  for  Irrigation 

Fields 138 

Sizes  and  Capacities  of  Drain  Pipes, 

Table  of 140 

Sizes  of  Sand,  Quantity  of  Sewage 

Purified  by  Different 92 

Sizes,   Separation  of  Sand  Grains 

into  Definite 198 

Sludge 12 

Sludge,  Burning  of 180 

Sludge,  C9mposition  of 179 

Sludge,  Disposal  of 179 

Sludge  in  Chemical    Precipitation 

Tanks 167 

Sludge  in  Deep  Water,  Disposal  of  180 
Sludge  in  Septic  Tanks,  Reduction 

of 

Sludge  on  Land,  Disposal  of 

Sludge  Pressing 

Sludge  Used  as  Fertilizer 180 

Sluice  Gate  Operating  Devices 51 

Sluice  Gates 50 

Sluice  Gates  Anchored  to  Masonry  51 

Sluice  Gates  and  Valves 49 

Sluice  Gates  Bolted  to  Pipe 51 


43 

181 


180 


SEWAGE    PURIFICATION    AND    DISPOSAL 


221 


PAGE 

Sluice  Gates  with  Thimbles  Set  in 

,   Concrete 51 

Soil  Moisture 130 

Soils 1 29. 

Soils,  Classification  of 129 

Soils,  Composition  of 129 

Soils,  Drainage  of 133 

Soils  for  Irrigation,  Preparing 133 

Soils,  Temperature  of 132 

Soils,  Voids  in 130 

Solid  Matter  in   Sewage,  Determi- 
nation of 25 

Solution,  Matter  in 9 

Space    in  Sand,  Determination    of 

Open 207 

Split-tile  Underdrains 74 

Sprinkler  Nozzle,  Columbus Ill 

Sprinkler  Nozzles 112 

Sprinkler  Systems,  Proportioning..  113 

Sprinklers,  Fixed 106 

Sprinklers,  Revolving 114 

Sprinkling  Filter  Beds,  Depth  of. . .  105 
Sprinkling  Filter    Beds,  Materials 

for 103 

Sprinkling  Filter  Beds,  Permanency 

of 119 

Sprinkling  Filter  Beds,  Size  of  Ma- 
terial for 104 

Sprinkling  Filter  Details 103 

Sprinkling  Filters 97 

Sprinkling  Filters,  Capacity  of 117 

Sprinkling  Filters,  Distributors  for  106 
Sprinkling  Filters,  Effect  of  Tem- 
perature on 119 

Sprinkling  Filters,  Efficiency  of. ...  117 

Sprinkling  Filters,  Examples  of 100 

Sprinkling  Filters,  Odors  from 118 

Sprinkling  Filters,  Period  of  Flow 

Through 106 

Sprinkling  Filtration,  Principles  of  97 

Staleness  of  Sewage 11 

Standardizing  Sieves 80 

Station,  Gasoline  Pumping 173 

Statistics  of  Sewage  Farms,  Table 

of 147 

Storm  Water,  Disposal  of 184 

Streams,  Disposal  of  Sewage  into.  177 
Streams,  Table  of  Safe  Dilution  of 

Sewage  in 178 

Strength  of  Sewage 12 

Sub-surface    Disposal    Plant,    Ex- 
ample of  a 150 

Sub-surface  Irrigation 146 

Sub-surface  Irrigation,  Application 

of,  to  Hillside 153 

Sub-surface    Irrigation,  Area  Re- 
quired for 156 

Sub-surface    Irrigation,    Distribu- 
ting Systems  for 151 

Sub-surface  Irrigation,  Fittings  for  152 
Sub-surface  Irrigation  Plants,  Op- 
eration of 158 

Sub-surface  Irrigation,  Principles 

of 146 

Sub-surface  Irrigation  Tiles,  Meth- 
od of  Laying 152 

Sub-surface      Irrigation,      Under- 
drains for 158 

Surface    Arrangement     of     Filter 

Beds 96 

Suspended  Matter 8 

System  for  Sub-surface  Irrigation, 

Distributing 151 

System  of  Irrigation,  Catchwork.  144 


PAGE 

System  of  Purification,  Deciding  on  195 
Systems  of  Underdrains  for  Irriga- 
tion Fields 135 

Systems  of  Filter  Underdrains 68 

Systems,  Proportioning  Sprinkler.  113 

Systems,  Sewerage 186 


Table  of  Capacities  of  Centrifugal 

Pumps 170 

Table  of  Capacity  of  Drains 71 

Table  of  Consumption  of  Water  in 

Cities 189 

Table  of  Mechanical   Composition 

of  Sands 93 

Table  of  Quantity  of  Sewage  Puri- 
fied by  Different  Sizes  of  Sand.. . .      92 
Table  of  Reagents  for  Precipitating 

Sewage 162 

Table  of   Safe  Dilution  of  Sewage 

in  Streams 178 

Table  of  Sizes    and   Capacities  of 

Drain  Pipes 140 

Table  of  Size  and  Rating  of  Sieves      79 
Table     of     Statistics     of     Sewage 

Farms 147 

Tank,  Description  of  a  Septic 36 

Tank  Details,  Septic 44 

Tank,  Detritus 44 

Tank,  Dosing 44 

Tank,  The  Septic 32 

Tanks,  Bacterial  and  Physical  Ac- 
tion in  Septic 32 

Tanks,  Capacity  of  Precipitation. . .     166 

Tanks,  Capacity  of  Septic 39 

Tanks,  Comparison  of  Covered  and 

Uncovered  Septic 85 

Tanks,  Construction  of  Septic 36 

Tanks,  Continuous  Flow  Chemical 

Precipitation 163 

Tanks,     Covered    and    Uncovered 

Septic 34 

Tanks,      Fill-and-Draw     Chemical 

Precipitation 165 

Tanks,    Reduction    of    Sludge     in 

Septic 43 

Tanks,  Shapes  and  Dimensions  of 

Septic 42 

Tanks,  Sludge  in  Chemical  Precipi- 
tation      167 

Tanks,Uniformity  of  Flow  Through 

Septic 41 

Tanks,  Velocity  of  Flow  Through 

Septic 40 

Tanks,  When  Septic,  are  Advisable    195 
Temperature,  Effect  of,  on  Contact 

Beds 12r 

Temperature,   Effect    of,  on   Inter- 
mittent Filtration 95 

Temperature,  Effect  of,  on  Sprink- 
ling Filters 119 

Temperature  of  Sewage 14 

Temperature  of  Sewage 25 

Temperature  of  Soils 132 

Test,  Fish 31 

Test,  Incubation 31 

Tests  for  Effluents 31 

Thimbles  Set   in   Concrete,  Sluice 

Gates  with 51 

Tide  Water,   Disposal    of    Sewage 

into 176 

Tile  Pipe  for  Underdrains 73 

Tile  Underdrains,  Perforated 73 


222 


SEWAGE    PURIFICATION    AND    DISPOSAL 


PAGE 

Tile  Underdrains,  Split 74 

Tiles,  Depth  of  Absorption 155 

Tiles,  Distance  Apart  of  Absorp- 
tion   155 

Tiles,  Grade  of  Absorption 155 

Tiles,  Method  of  Laying  Sub-sur- 
face Irrigation 152 

Tiles,  Size  of  Absorption 155 

Traveling  Distributor,  Autoinatic.  115 
Treatment  of  Sewage,  Antiseptic. .  160 
Turbidity,  Odor  and  Color  of  Sew- 
age   24 

u 

Uncovered    and    Covered     Septic 

Tanks.. 84 

Uncovered    and    Covered     Septic 

Tanks,  Comparison  of 85 

Underdrains,  Brick 78 

Underdrains,  Depth  of  Irrigation. .  189 
Underdrains,    Distance    Between 

Irrigation 189 

Underdrains,  Grades  for  Irrigation  141 

Underdrains,  Filter. 68 

Underdrains  for  Irrigation  Fields, 

Size  of 188 

Underdrains  for  Irrigation  Fields, 

Systems  of 185 

Underdrains,  Foimula  of  Propor- 
tioning   59 

Underdrains,  Perforated  Tile 73 

Underdrains  for  Sub-surface  Irriga- 
tion...                                                 .  158 


PAGE 

Underdrains,  Split-tile 74 

Underdrains,  Systems  of  Filter 68 

Underdrains,  Tile  Pipe  for 7:5 

Uniformity  Coefficient  of  Sand 76 

Uniformity  Coefficient  of  Sand, 

Calculating  the 78 

Uniformity  of  Flow  Through  Septic 

Tanks 41 


Valves  and  Gates,  Sluice 

Valves,  Gate 

Variation  in  the  Flow  of  Sewage. . . 
Velocity  of  Flow  Through  Septic 

Tanks 

Voids  in  Soils 

w 


49 
52 
13 

40 
130 


104 


1FO 
184 


210 

207 


Wastes,  Manufacturing .... 

Water  and  Air  Capacity  of  Sands. . 

Water,  Disposal  of  Sludge  in  Deep 

Water,  Disposal  of  Storm 

Water  in  Sand  and  Gravels,  Fric- 
tion of 

Water  in  Sand,  Determination  ot. . . 

Water,  Rate  at  which,  Passes 
Through  Gravels 213 

Water,  Rate  at  which,  Passes 
Through  Sands 212 

Water,  Table  of  Consumption  of,  in 
Cities 189 

Weighing  Sand,  Scales  for 81 

Works,  Location  for  Purification. . .    193 


Chasmar-Winchell  Press     New  York  and  Pittsburgh 


UNIVERSITY  OP  CALIFORNIA  LIBRARY 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW   " 


16  1915 


m."i  11 

MAY  1  191? 


19NovDEAD 


YC  13237 


